I was thinking of something more integrated - an actual vBulletin mod. Something which uses vBulletin's database for accounts, which integrates elements of usage into vBulletin user profiles, possibly the groups (Campaign Manager) system and so on. Totally depends on their business model, of course; that would work really well for a product producer with the product licensing route, but not so well with the web traffic route.
Yeah, I touched on that in the post over there, which has brought Riley back here:
Sign In - Roll20 Community
As a minimum, just having a button to launch Roll20 would help. But in reality, we'd really want it tied into ENworld user accounts and friendslists and the game seeker/scheduler
One benefit of the scheduler, is once an event was schedule in ENWorld, the hyperlink could be displayed in the calendar event, which only joined/invited users would see. The system could send out the email notification that the game was going to start in 10 minutes, and thus contain the link to join the Roll20 game.
Thus, the GM doesn't have to really manage that part, ENworld would do it for him.
I'm wary to use the term vBulletin mod. I have no doubt the Roll20 app/API would need to talk to certain vBulletin resources (users, friends, calendar, etc), but roll20 is a full screen app. It is not something you'd want running as a sub-component on the screen with the usual headers, footers and advertisements taking up space.
It is probable that it could run within the vBulletin content space, but it's a highly visual app, which means full screen, maximized window is best for fitting everything in that it needs, and external wrappers causing scrollbars, etc would diminish that.