Merging Users:
I didn't read all the topics, so maybe this has been mentioned, but here is a possible work around for letting current members set up their face book accounts.
1: read:
http://www.vbulletin.com/docs/html/users_merge
2: Get current members with facebook to log in with their facebook accounts and us a nick name as 'username2' or something.
3. Then merge this user with their current forum username.
I havent tried it yet, but I assume that it will work.
* Edit, I found Jedi's post where he said that current nick name can be used. I have to try this out.