For a serious community (i.e., one with a business model), the pricing differences in vBulletin versions are inconsequential. What counts is the combo of reliability, scalability, and continued development.
As far as crawlability and SEO, vBulletin is getting better out of the box and vBSEO does the rest.
One issue: WebmasterWorld has been popular (IMO) in part because of its speed. It's simple, and it's very fast. To duplicate this level of performance on vBulletin would take (my guess, I don't know all of the details) a significant hardware upgrade. I've got a forum that might be somewhat comparable in size that now runs in a cloud setup but would likely take a full rack of servers to handle. vB is hardware intensive, no getting around that.
Another issue: even with redundant or optional stuff turned off, vB will present the users with a more complex page than BestBBS. Many people like WebmasterWorld's minimalist look. (On my first vB implementation, I deleted gobs of code but caught hell from the admin responsible for upgrading it later.)
Having said all that, I'm a fan of vB because of the ongoing development investment by the company itself and the very active plugin community. The upside of "bloat" is that if you need a feature it is likely there already. If it's not, someone has probably written a plugin for it. Some of the Web's biggest forums run vB, which says a lot.