I ended up doing the same thing as
Dekard. Go to home page of Joomla! site, right click, then view source.
On our particular setup, I copied everything below the <title></title> down to right above the </head> tag and copied that into the bottom of my vB headinclude template.
Then going back to the html view source from the J! page I copied everything below the <body> tag down to just above the <div id=main-content> and pasted that into my vB header template in place of the the whole Logo section. IOW delete the logo section entirely from the header template and paste in the html copied from the view source.
On our particular modified Rockettheme template this creates a static snapshot of your Joomla header. The menu works perfectly, and the visual integration is nearly perfect. We'll have to update it to refelect any changes made to menu items etc, and there are a handfull of validation errors I still need to sort out, but it's *fast* loading and looks beautiful.
The problem with the wrapper is that it totally screwes up your urls from the forum, as they are now all Joomla URLs. So if you have an established site with any SERPs in Google etc, you'll lose it all. Things like VBSEO will be essentially useless to you as well.
Anyway, I'm not a developer, and please don't ask for help with the above. It's just an account of what's worked for me, you'll have to adapted it to your particular vB and J! template.
James