BuddyPress.org

Do a search for BBPress import from vBulletin and see what turns up. Right now BP just uses BBpress as the forum.

Forums are tied to groups in BP, unless you use a standalone BBPress install.

There is no bridge for vBulletin right now, but as BBPress is getting redone I think the idea is to make the integration more open so someone could build a vBulletin bridge if there was demand for it.