I am intensely interested in this area as well, and so far the best I've seen is for Joomla/Mambo-to-vBulletin. It's called vBridge and you can read about it
here. The single-database version is already released, and they are very close to releasing a multi-database version.
As I'm not 100% sold on the Joomla/Mambo platform just yet (still a wee bit shy on features I need) I would be VERY interested to hear if anyone else has come across a strong alternative that offers the following features at a minimum (ideally tied to vBulletin also):
- Shared user membership with vBulletin (of course)
- Robust community event calendar
- Decent commerce system
- Decent other usual stuff (blogs, article submission, etc.)
- Decent templates
- Easy to set up for someone still getting their feet wet with this stuff like I am
I don't know of any other CMS that can currently integrate membership with vbulletin, and I have a mere two weeks left on a deadline.