Forgive me if this seems like nit-picking, but there is a difference between C++ projects and C++/CLI projects.
A reference to a project that creates a C++ dll cannot be added to a VB.net project.
If one creates a VC++ CLR class library (i.e. C++/CLI) or adds /clr for common language runtime support to a Win32 Dll project then adding a reference is possible.