I would suggest both. I use vbulletin (plus some minor hacks - welcome headers, quick account switch, vbspell, support forums, latest paid subscribers and so on) and vbDrupal -
http://www.vbdrupal.org. I was going to click 'install' for vbdrupal here, but for some reason I can't access the link in Tamarain's signature.
Anyway, a hell of a lot of drupal modules will work with vbdrupal, as well as the themes. If you want the vb look, it takes on the vbulletin templates pretty seemlessly, provides blogs, stories...etc etc.
My site is a bit sketchy right now:
http://www.alleria.com but we have over a 1000 pages of content, and our user base is huge. We recently upgraded to 4.7.4.3 vbDrupal and 3.6.4 of vBulletin, so I have a lot of work to do to get it running seemlessly again - but its power, with things like views and actions modules, and ...cck and everything else just makes any other CMS addon look ridiculous.
And the best part? the forums themselves stay nice and safe