Here's what you guys should do if you have access to a php programmer.
IN the vbulletin database you add a new table called old_favorites that has a user ID, thread URL, and thread title (user id and thread url are combo primary key). You write a couple line script that loops through each user ID in UBB and grabs their favorite thread IDs and titles.
You convert the thread ID into the proper archived URL location. For example a UBB thread ID may say 777634 so you'd convert it to
http://archiveserver.twoplustwo.com/...read/ID/777623 (or whatever archive URL you guys are using) and insert that and the thread title into the old_favorites table for that persons vbulletin user ID.
Then in vBulletin you add a few lines to the "my home" php page that loops through the old_favorites field and displays a table of links to their old favorites.
It probably took me longer to type all this out than it would to develop the script. You just have to make sure each person in UBB has the same username in vbulletin so you can link their user IDs.