[BSMedia]

You could do this with the built in subscriptions and promotion system.

Deny "guests and registered users" access to everything, except maybe one forum promoting your product or service. Then after that, send them the link to subscribe and once they pay using the built in vBulletin subscription system, they'll automatically be updated to the usergroup that has access to everything.

You could even do tierd memberships if you wanted. Say if someone only pays $10 they get half the access that someone who paid $20 does