External User Integration: Recommendations?

[sde]

By the time the user gets to vBulletin, they are already verified and we know who they are.

I'd like the vb account to automatically be created and logged in. Upon re-visiting the forum, I'd like to automatically log them in (since I already have their site-wide user data).

Any recommendations on mods or hacks?

[Dismounted]

Look at how vBulletin logs someone in (see login.php/functions_login.php).