Can a third party login/signup be integrated?
 

My client already has a signup and a pay wall for subscriptions to their service. They would like to install something akin to vbulletin to be accessed by paid members once they are logged in.

Is this doable?

How do I turn off the login/signup on vbulletin such that their sites signup form inserts directly into vbulletin, and you don't have to log in to both?