Found and answer - specifically a workaround
https://developercommunity.visualstudio.com/content/problem/22031/unusable-vc-build-tools-2015-installed-with-vs2017.html
Solution by Joseph Shen ยท Jun 21, 2017 at 04:41 PM
I read the vcvarsall.bat source code, found that the VCVarsQueryRegistry.bat will try to enum the include dir for Windows SDK version. after install VS2017, Windows SDK 10 will be found. but this not the same as VS2015 install only status.
My fix is call with version specified like this:
%comspec% /k ""C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat"" x86 8.1
%comspec% /k ""C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat"" amd64 8.1
please note the 8.1 been specified!!