Auto-login/register on site that has existing login/registration system?

[redefinery]

I'm trying to integrate vBulletin as the forums for a restricted area of my site. The site already has its own login and registration system (php/mysql), so I'm trying to find a solution so that when a user registers on my site, he/she also gets registered as a user in vBulletin. Then, when the user logs in to my site and then accesses the vBulletin forums, he/she gets automatically logged in.

Is there a hack or some code that will allow me to do this?

If not, what are the minimum required fields to register a new user? And, if the user is already logged in (i.e., a username and password is available), what do I need to set (cookies, session data) or call in the vBulletin code to mimic a logged-in user?

Thanks,
Jeff