[ElfMage]

Inserting a record in the session table and setting the right cookies should work. But, you need to take into account the sessionhash that vB generates based on the user's IP and browser's user agent (md5'ed).

Alternatively, you could do a server-side (or client-side clear gif) request to vBulletin's login.php, just like the login form does, passing the user's username and md5 password (you could send clear password as well, but this would be a security hole).

Take a look at CES Cookie Cutter. It doesn't do what you need, but it shows the use of a clear-gif client side to transfer vB's cookies/login from one domain to a different domain.