yes it can be done, ditch your current website membership scheme (if you have one) and put all the data into the vbulletin tables, then just require config.php on each page and use the same validation scheme as vb (will be sessions soon so you can use them), adding new fields to the member database is just a matter of using myphpadmin and should be no problem.

others have done it i know, a search here will probably find them

i will tell you more about how it works i have a hack done that is going to merge links SQL and vbulletin together (not being made by me though)