The better kind of forum for SEO is where:
A- Your links are dofollow. (Obviously.)
B- Your forum profile is visible to the search engines, and your posts have a dofollow link to that profile.
B is important since your forum posts will channel internal
PR to your forum profile.
Not all vBulletin boards are setup like this. Without it, you will just have hundreds of extremely low quality links. Hundreds of low quality links is worth less than a single quality link.
A way around this might be to have your signature link to one of your posts (or a post in which you've commented in; preferably relevant to your site).