diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index 8a67335b3e..975b02252e 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -581,8 +581,6 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		// Show vendor-expanded paths in listing
 		p.TestImports = p.Resolve(p.TestImports)
 		p.XTestImports = p.Resolve(p.XTestImports)
-		p.TestEmbedFiles = p.ResolveEmbed(p.TestEmbedPatterns)
-		p.XTestEmbedFiles = p.ResolveEmbed(p.XTestEmbedPatterns)
 		p.DepOnly = !cmdline[p]
 
 		if *listCompiled {
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index a1be074f6a..92dd794871 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -1807,7 +1807,7 @@ func (p *Package) load(ctx context.Context, path string, stk *ImportStack, impor
 	stk.Push(path)
 	defer stk.Pop()
 
-	p.EmbedFiles, p.Internal.Embed, err = p.resolveEmbed(p.EmbedPatterns)
+	p.EmbedFiles, p.Internal.Embed, err = resolveEmbed(p.Dir, p.EmbedPatterns)
 	if err != nil {
 		setError(err)
 		embedErr := err.(*EmbedError)
@@ -1932,17 +1932,20 @@ func (e *EmbedError) Unwrap() error {
 }
 
 // ResolveEmbed resolves //go:embed patterns and returns only the file list.
-// For use by go list to compute p.TestEmbedFiles and p.XTestEmbedFiles.
-func (p *Package) ResolveEmbed(patterns []string) []string {
-	files, _, _ := p.resolveEmbed(patterns)
-	return files
+// For use by go mod vendor to find embedded files it should copy into the
+// vendor directory.
+// TODO(#42504): Once go mod vendor uses load.PackagesAndErrors, just
+// call (*Package).ResolveEmbed
+func ResolveEmbed(dir string, patterns []string) ([]string, error) {
+	files, _, err := resolveEmbed(dir, patterns)
+	return files, err
 }
 
 // resolveEmbed resolves //go:embed patterns to precise file lists.
 // It sets files to the list of unique files matched (for go list),
 // and it sets pmap to the more precise mapping from
 // patterns to files.
-func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[string][]string, err error) {
+func resolveEmbed(pkgdir string, patterns []string) (files []string, pmap map[string][]string, err error) {
 	var pattern string
 	defer func() {
 		if err != nil {
@@ -1953,6 +1956,7 @@ func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[stri
 		}
 	}()
 
+	// TODO(rsc): All these messages need position information for better error reports.
 	pmap = make(map[string][]string)
 	have := make(map[string]int)
 	dirOK := make(map[string]bool)
@@ -1966,7 +1970,7 @@ func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[stri
 		}
 
 		// Glob to find matches.
-		match, err := fsys.Glob(p.Dir + string(filepath.Separator) + filepath.FromSlash(pattern))
+		match, err := fsys.Glob(pkgdir + string(filepath.Separator) + filepath.FromSlash(pattern))
 		if err != nil {
 			return nil, nil, err
 		}
@@ -1977,7 +1981,7 @@ func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[stri
 		// then there may be other things lying around, like symbolic links or .git directories.)
 		var list []string
 		for _, file := range match {
-			rel := filepath.ToSlash(file[len(p.Dir)+1:]) // file, relative to p.Dir
+			rel := filepath.ToSlash(file[len(pkgdir)+1:]) // file, relative to p.Dir
 
 			what := "file"
 			info, err := fsys.Lstat(file)
@@ -1990,13 +1994,13 @@ func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[stri
 
 			// Check that directories along path do not begin a new module
 			// (do not contain a go.mod).
-			for dir := file; len(dir) > len(p.Dir)+1 && !dirOK[dir]; dir = filepath.Dir(dir) {
+			for dir := file; len(dir) > len(pkgdir)+1 && !dirOK[dir]; dir = filepath.Dir(dir) {
 				if _, err := fsys.Stat(filepath.Join(dir, "go.mod")); err == nil {
 					return nil, nil, fmt.Errorf("cannot embed %s %s: in different module", what, rel)
 				}
 				if dir != file {
 					if info, err := fsys.Lstat(dir); err == nil && !info.IsDir() {
-						return nil, nil, fmt.Errorf("cannot embed %s %s: in non-directory %s", what, rel, dir[len(p.Dir)+1:])
+						return nil, nil, fmt.Errorf("cannot embed %s %s: in non-directory %s", what, rel, dir[len(pkgdir)+1:])
 					}
 				}
 				dirOK[dir] = true
@@ -2027,7 +2031,7 @@ func (p *Package) resolveEmbed(patterns []string) (files []string, pmap map[stri
 					if err != nil {
 						return err
 					}
-					rel := filepath.ToSlash(path[len(p.Dir)+1:])
+					rel := filepath.ToSlash(path[len(pkgdir)+1:])
 					name := info.Name()
 					if path != file && (isBadEmbedName(name) || name[0] == '.' || name[0] == '_') {
 						// Ignore bad names, assuming they won't go into modules.
diff --git a/src/cmd/go/internal/load/test.go b/src/cmd/go/internal/load/test.go
index 178f257f4b..eb8aef3ee2 100644
--- a/src/cmd/go/internal/load/test.go
+++ b/src/cmd/go/internal/load/test.go
@@ -124,7 +124,7 @@ func TestPackagesAndErrors(ctx context.Context, p *Package, cover *TestCover) (p
 		imports = append(imports, p1)
 	}
 	var err error
-	p.TestEmbedFiles, testEmbed, err = p.resolveEmbed(p.TestEmbedPatterns)
+	p.TestEmbedFiles, testEmbed, err = resolveEmbed(p.Dir, p.TestEmbedPatterns)
 	if err != nil && ptestErr == nil {
 		ptestErr = &PackageError{
 			ImportStack: stk.Copy(),
@@ -147,7 +147,7 @@ func TestPackagesAndErrors(ctx context.Context, p *Package, cover *TestCover) (p
 		}
 		p.XTestImports[i] = p1.ImportPath
 	}
-	p.XTestEmbedFiles, xtestEmbed, err = p.resolveEmbed(p.XTestEmbedPatterns)
+	p.XTestEmbedFiles, xtestEmbed, err = resolveEmbed(p.Dir, p.XTestEmbedPatterns)
 	if err != nil && pxtestErr == nil {
 		pxtestErr = &PackageError{
 			ImportStack: stk.Copy(),
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index e42ff42fbd..d3ed9e00e2 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -7,7 +7,9 @@ package modcmd
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
+	"go/build"
 	"io"
 	"io/fs"
 	"os"
@@ -19,7 +21,9 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/imports"
+	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
@@ -182,19 +186,76 @@ func moduleLine(m, r module.Version) string {
 }
 
 func vendorPkg(vdir, pkg string) {
+	// TODO(#42504): Instead of calling modload.ImportMap then build.ImportDir,
+	// just call load.PackagesAndErrors. To do that, we need to add a good way
+	// to ignore build constraints.
 	realPath := modload.ImportMap(pkg)
 	if realPath != pkg && modload.ImportMap(realPath) != "" {
 		fmt.Fprintf(os.Stderr, "warning: %s imported as both %s and %s; making two copies.\n", realPath, realPath, pkg)
 	}
 
+	copiedFiles := make(map[string]bool)
 	dst := filepath.Join(vdir, pkg)
 	src := modload.PackageDir(realPath)
 	if src == "" {
 		fmt.Fprintf(os.Stderr, "internal error: no pkg for %s -> %s\n", pkg, realPath)
 	}
-	copyDir(dst, src, matchPotentialSourceFile)
+	copyDir(dst, src, matchPotentialSourceFile, copiedFiles)
 	if m := modload.PackageModule(realPath); m.Path != "" {
-		copyMetadata(m.Path, realPath, dst, src)
+		copyMetadata(m.Path, realPath, dst, src, copiedFiles)
+	}
+
+	ctx := build.Default
+	ctx.UseAllFiles = true
+	bp, err := ctx.ImportDir(src, build.IgnoreVendor)
+	// Because UseAllFiles is set on the build.Context, it's possible ta get
+	// a MultiplePackageError on an otherwise valid package: the package could
+	// have different names for GOOS=windows and GOOS=mac for example. On the
+	// other hand if there's a NoGoError, the package might have source files
+	// specifying "// +build ignore" those packages should be skipped because
+	// embeds from ignored files can't be used.
+	// TODO(#42504): Find a better way to avoid errors from ImportDir. We'll
+	// need to figure this out when we switch to PackagesAndErrors as per the
+	// TODO above.
+	var multiplePackageError *build.MultiplePackageError
+	var noGoError *build.NoGoError
+	if err != nil {
+		if errors.As(err, &noGoError) {
+			return // No source files in this package are built. Skip embeds in ignored files.
+		} else if !errors.As(err, &multiplePackageError) { // multiplePackgeErrors are okay, but others are not.
+			base.Fatalf("internal error: failed to find embedded files of %s: %v\n", pkg, err)
+		}
+	}
+	embedPatterns := str.StringList(bp.EmbedPatterns, bp.TestEmbedPatterns, bp.XTestEmbedPatterns)
+	embeds, err := load.ResolveEmbed(bp.Dir, embedPatterns)
+	if err != nil {
+		base.Fatalf("go mod vendor: %v", err)
+	}
+	for _, embed := range embeds {
+		embedDst := filepath.Join(dst, embed)
+		if copiedFiles[embedDst] {
+			continue
+		}
+
+		// Copy the file as is done by copyDir below.
+		r, err := os.Open(filepath.Join(src, embed))
+		if err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		if err := os.MkdirAll(filepath.Dir(embedDst), 0777); err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		w, err := os.Create(embedDst)
+		if err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		if _, err := io.Copy(w, r); err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		r.Close()
+		if err := w.Close(); err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
 	}
 }
 
@@ -207,14 +268,14 @@ var copiedMetadata = make(map[metakey]bool)
 
 // copyMetadata copies metadata files from parents of src to parents of dst,
 // stopping after processing the src parent for modPath.
-func copyMetadata(modPath, pkg, dst, src string) {
+func copyMetadata(modPath, pkg, dst, src string, copiedFiles map[string]bool) {
 	for parent := 0; ; parent++ {
 		if copiedMetadata[metakey{modPath, dst}] {
 			break
 		}
 		copiedMetadata[metakey{modPath, dst}] = true
 		if parent > 0 {
-			copyDir(dst, src, matchMetadata)
+			copyDir(dst, src, matchMetadata, copiedFiles)
 		}
 		if modPath == pkg {
 			break
@@ -282,7 +343,7 @@ func matchPotentialSourceFile(dir string, info fs.DirEntry) bool {
 }
 
 // copyDir copies all regular files satisfying match(info) from src to dst.
-func copyDir(dst, src string, match func(dir string, info fs.DirEntry) bool) {
+func copyDir(dst, src string, match func(dir string, info fs.DirEntry) bool, copiedFiles map[string]bool) {
 	files, err := os.ReadDir(src)
 	if err != nil {
 		base.Fatalf("go mod vendor: %v", err)
@@ -294,11 +355,14 @@ func copyDir(dst, src string, match func(dir string, info fs.DirEntry) bool) {
 		if file.IsDir() || !file.Type().IsRegular() || !match(src, file) {
 			continue
 		}
+		copiedFiles[file.Name()] = true
 		r, err := os.Open(filepath.Join(src, file.Name()))
 		if err != nil {
 			base.Fatalf("go mod vendor: %v", err)
 		}
-		w, err := os.Create(filepath.Join(dst, file.Name()))
+		dstPath := filepath.Join(dst, file.Name())
+		copiedFiles[dstPath] = true
+		w, err := os.Create(dstPath)
 		if err != nil {
 			base.Fatalf("go mod vendor: %v", err)
 		}
diff --git a/src/cmd/go/testdata/script/embed.txt b/src/cmd/go/testdata/script/embed.txt
index 710968feca..2ad799b7a7 100644
--- a/src/cmd/go/testdata/script/embed.txt
+++ b/src/cmd/go/testdata/script/embed.txt
@@ -3,6 +3,14 @@ go list -f '{{.EmbedPatterns}}'
 stdout '\[x\*t\*t\]'
 go list -f '{{.EmbedFiles}}'
 stdout '\[x.txt\]'
+go list -test -f '{{.TestEmbedPatterns}}'
+stdout '\[y\*t\*t\]'
+go list -test -f '{{.TestEmbedFiles}}'
+stdout '\[y.txt\]'
+go list -test -f '{{.XTestEmbedPatterns}}'
+stdout '\[z\*t\*t\]'
+go list -test -f '{{.XTestEmbedFiles}}'
+stdout '\[z.txt\]'
 
 # build embeds x.txt
 go build -x
@@ -58,6 +66,22 @@ import "embed"
 //go:embed x*t*t
 var X embed.FS
 
+-- x_test.go --
+package p
+
+import "embed"
+
+//go:embed y*t*t
+var Y string
+
+-- x_x_test.go --
+package p_test
+
+import "embed"
+
+//go:embed z*t*t
+var Z string
+
 -- x.go2 --
 package p
 
@@ -69,6 +93,8 @@ var X embed.FS
 -- x.txt --
 hello
 
+-- y.txt --
+-- z.txt --
 -- x.txt2 --
 not hello
 
diff --git a/src/cmd/go/testdata/script/mod_vendor_embed.txt b/src/cmd/go/testdata/script/mod_vendor_embed.txt
new file mode 100644
index 0000000000..be114159a1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_embed.txt
@@ -0,0 +1,179 @@
+go mod vendor
+cmp vendor/example.com/a/samedir_embed.txt a/samedir_embed.txt
+cmp vendor/example.com/a/subdir/embed.txt a/subdir/embed.txt
+cmp vendor/example.com/a/subdir/test/embed.txt a/subdir/test/embed.txt
+cmp vendor/example.com/a/subdir/test/xtest/embed.txt a/subdir/test/xtest/embed.txt
+
+cd broken_no_matching_files
+! go mod vendor
+stderr 'go mod vendor: pattern foo.txt: no matching files found'
+
+cd ../broken_bad_pattern
+! go mod vendor
+stderr 'go mod vendor: pattern ../foo.txt: invalid pattern syntax'
+
+# matchPotentialSourceFile prunes out tests and unbuilt code.
+# Make sure that they are vendored if they are embedded files.
+cd ../embed_unbuilt
+go mod vendor
+cmp vendor/example.com/dep/unbuilt.go dep/unbuilt.go
+cmp vendor/example.com/dep/dep_test.go dep/dep_test.go
+! exists vendor/example.com/dep/not_embedded_unbuilt.go
+! exists vendor/example.com/dep/not_embedded_dep_test.go
+-- go.mod --
+module example.com/foo
+go 1.16
+
+require (
+	example.com/a v0.1.0
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+)
+-- foo.go --
+package main
+
+import (
+	"fmt"
+
+	"example.com/a"
+)
+
+func main() {
+    fmt.Println(a.Str())
+}
+-- a/go.mod --
+module example.com/a
+-- a/a.go --
+package a
+
+import _ "embed"
+
+//go:embed samedir_embed.txt
+var sameDir string
+
+//go:embed subdir/embed.txt
+var subDir string
+
+func Str() string {
+	return sameDir + subDir
+}
+-- a/a_test.go --
+package a
+
+import _ "embed"
+
+//go:embed subdir/test/embed.txt
+var subderTest string
+-- a/a_x_test.go --
+package a_test
+
+import _ "embed"
+
+//go:embed subdir/test/xtest/embed.txt
+var subdirXtest string
+-- a/samedir_embed.txt --
+embedded file in same directory as package
+-- a/subdir/embed.txt --
+embedded file in subdirectory of package
+-- a/subdir/test/embed.txt --
+embedded file of test in subdirectory of package
+-- a/subdir/test/xtest/embed.txt --
+embedded file of xtest in subdirectory of package
+-- broken_no_matching_files/go.mod --
+module example.com/broken
+go 1.16
+
+require (
+	example.com/brokendep v0.1.0
+)
+
+replace (
+	example.com/brokendep v0.1.0 => ./brokendep
+)
+-- broken_no_matching_files/f.go --
+package broken
+
+import _ "example.com/brokendep"
+
+func F() {}
+-- broken_no_matching_files/brokendep/go.mod --
+module example.com/brokendep
+go 1.16
+-- broken_no_matching_files/brokendep/f.go --
+package brokendep
+
+import _ "embed"
+
+//go:embed foo.txt
+var foo string
+-- broken_bad_pattern/go.mod --
+module example.com/broken
+go 1.16
+
+require (
+	example.com/brokendep v0.1.0
+)
+
+replace (
+	example.com/brokendep v0.1.0 => ./brokendep
+)
+-- broken_bad_pattern/f.go --
+package broken
+
+import _ "example.com/brokendep"
+
+func F() {}
+-- broken_bad_pattern/brokendep/go.mod --
+module example.com/brokendep
+go 1.16
+-- broken_bad_pattern/brokendep/f.go --
+package brokendep
+
+import _ "embed"
+
+//go:embed ../foo.txt
+var foo string
+-- embed_unbuilt/go.mod --
+module example.com/foo
+go 1.16
+
+require (
+	example.com/dep v0.1.0
+)
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+)
+-- embed_unbuilt/foo.go --
+package a
+
+import _ "example.com/dep"
+
+func F() {}
+-- embed_unbuilt/dep/go.mod --
+module example.com/dep
+go 1.16
+-- embed_unbuilt/dep/dep.go --
+package dep
+
+import _ "embed"
+
+//go:embed unbuilt.go
+var unbuilt string
+
+//go:embed dep_test.go
+var depTest string
+-- embed_unbuilt/dep/unbuilt.go --
+// +build ignore
+
+package dep
+-- embed_unbuilt/dep/not_embedded_unbuilt.go --
+// +build ignore
+
+package dep
+-- embed_unbuilt/dep/dep_test.go --
+package dep
+-- embed_unbuilt/dep/not_embedded_dep_test.go --
+package dep