Use same version of MSBuild as solution file (VS 2015 / 14.0).

vars.bat

@@ -1,7 +1,7 @@
 @echo off
-@echo This build environment requires .NET Framework v4.0
+@echo This build environment requires MSBuild 14.0
 @echo To build use 'msbuild'
 
-set path=%SystemRoot%\Microsoft.NET\Framework\v4.0.30319;%path%
+set PATH=%ProgramFiles(x86)%\MSBuild\14.0\bin;%PATH%
 set path=%~dp0tools\NuGet;%path%
 set path=%~dp0tools\WiX;%path%