Tie in to existing user db

[Barbarian]

We have an existing site with a complete login system up and running(with about 40k registered users). Basically, I'd like to integrate vBulletin into this existing membership. I'd like them to login through our existing site, and have it so they're also logged in vBulletin so that they don't have to have separate logins.

Anybody have any experience with this?