I have a Visual Studio 2008 C++ project that has support for using multiple CPUs/cores when compiling. In the VCPROJ file I see this:
Tools > Options > Projects and Solutions > Build and Run > maximum number of parallel project builds
To enable /MP option you could add it to Project Settings->C/C++->Command Line|Additional options. This is the only way to switch it on in vcproj.
Project Settings->C/C++->Command Line|Additional options