I used to run an Invision Board with YAUWS portal. I've since upgraded my forum to vBulletin 3.5 (today) and am looking for some good, stable alternatives.
Check out the Drupal integration Tamarian did right here int the Extensions forum. Check out subdreamer.com it is pretty good, its what I use right now. Mkportal isn't bad. vbadvanced is also quite good.
And there is a Mambo/Joomla to vBulletin bridge, but I've heard very mixed stories on it.