Hi,
I'm not sure what you're looking for. Let's say Site A runs a vB, Site B runs a UBB, and Site C runs nada.
The UBB has 3 forums. One discussing matters regardng site B but has two other forums referring to Site A and Site C. When clicking the forum to Site a, you are redirected to the vB site and when clicking on the Site C forum you are redirected to Site C. Is this the kind of functionality you want?
qasic