Why not just write your own batch ('buildit.bat') file that does:
call vcvarsall.bat
nmake
(or whatever the command to build in the VS command prompt is...)
And then your command to build is:
C:/full/path/to/buildit.bat
It's presumably in a Windows-specific chunk of your CMakeLists anyway,
so that should work fairly simply.
HTH,
David C.