Depends on your site design and how much time you're willing to put in to modifying the forum skin but
vBulletin is pretty flexible.
As an example of skinned vBulletin, I've just been helping a good friend of mine with a music news and review site, I was creating a few PHP scripts for her,
On The Barrier is the main site and
Music Social Network is the forum social portion of the site. It's only been there a couple of days which is why it's a little sparse.
In this case the forum is on a sub-domain but it doesn't have to be and could have been in a folder on the main domain, we had a reason for keeping it separate in this case. It is possible to have it match the main site more closely, but there are some extra menus the forum needs so you do need to adjust the design to get them all in.
Only thing with vBulletin is it's table based.