hi
i use the oscommerce since 6 years and i can tell that i know this engine (oscommerce)
to make a good brigde you need make this:
1) changing password function in vbulletin - and use from oscommerce (its better than in vb)
2) add some queries to both registering pages
3) make a one mod in activity link (add one param to check that user wants to have shopping feature)
4) in register form you should put the checkbox which tell to the script that user wants to be forum member (and vice versa)
thats all
as additional stuff you can add the "your basket" in forum navibar and of course link to the store.
i thinking now about a special promotion (in usergroup) for users which buy more than one time or leave for me some ammount of $$$
there is a many ways to make this a one system with really good feature. if you wants i can in next week write more description about this bridge.