[Marco van Herwaarden]

Using vBulletin without its own user database, is almost impossible (as vBulletin needs a lot more then only username, password, etc...). Verifying a login using an external database (or setting the cookie from an external page) is however possible, but might not be the most flexible way to go.

I can not give a more precise answer as it will depend greatly on the complexitiy of your current external code (1 or the other will need adjustments, the question is what is easier to adjust, vB or your custom code).