vBulletin DOES store a user ID for you, at least internally. It's in the "userid" field of the "user" table.
You can get the userID of a person by clicking their "Profile" link and looking at the "userid=" part of the URL.
For example, I am user #1196.
I don't think anyone has #666, because the following URL says "Invalid user specified":
http://www.eunuch.org/vbulletin/member. ... userid=666