You can always use the "wrapper" feature of Joomla too. It will wrap (ie. IFRAME) any external site into your Joomla site thereby displaying Joomla's header and footer. Some tweaking of your vBulletin style may be necessary to get it to look right, but it should accomplish what you're looking for I'd think.
How does this work? I have J1.5 installed, trying to test it with JFusion and my vBulletin site. JFusion appears to be working fine, but I just can't figure out how to get Joomla to put my site in a wrapper on a link. Ugh.