We used to use Tufat scripts which allowed users to chat but they had to actually log into it even though it authenticated automatically with vbulletin.
Now we are using cometchat in which the chat bar is ALLWAYS open and idicates when their vbulletin friends are online. Our install was modified by them to ONLY show logged in users. Our forum has taken off since we did this as it becomes a LIVE forum
Look at
www.cometchat.com