What is the best way to hook in an external login to vBulletin.

In the past we hacked the code to update a User table on another DB to keep every thing in Sync etc., but I am wondering if there is not a better way to do this through hooks or some kind of module architecture. Basically we would like to keep all user info in a different database.

I'm sure someone must have done this before. Any suggestions on how to approach it or potential pitfalls?

Is this something that could be done within the 'product' plug-in system or will it required more serious hacking?

Thanks!

everything can be done with the product system, *hail product system* :p.