Replacing the registration and login system

I've developed a (not-yet-launched) site that its own user-registration system, so people on this site already have usernames and passwords. The client wants forums, and vBulletin was the obvious choice, but I want to somehow integrate the current user registration with it so that people don't have to register once for the site, and again for the forums - as well as log in twice.

So my current plan is to insert a row into the user table upon registration to the site with the minimum amount of info required to be a user. And then somehow set a cookie that looks like the one vB sets on login so that people will be automatically logged in when they go to the forum section of the site.

Seems like it should be straightfoward(ish), but I thought I'd post here to see if anyone's done something like this, or knows of any pitfalls I might run into.