I use Joomla as my main front page and vBulletin for the forum. All registration is done through vBulletin as I use Joomla to display images of puzzle magazines and post information on new editions available for download from forum section.
I use iJoomla mod for layout of main site,
www.magazine.punzhu.com.
vBulletin forum and magazines are hyperlinked from main site.
I've used this combination for two years and have had no problems, but then I don't have any reason to link vBulletin members to Joomla as I'm not running a typical forum community.
My site is not being used by members to build a puzzle community as 100% of visitors/members come to download the free puzzle magazines. So having the ability to link memberships between Joomla and vBulletin is not important for my site at the moment.
In fact, later on during this long weekend I'm going to remove the registration requirement for downloading the free puzzle magazines. Registration will only be required if a visitor wants to post/reply to something on the forum board. This will allow members to form a community if they wish while not impeding the access to free puzzle magazines (as magazines can be copied and distributed to others there is no direct need for registration, other than building a community).