The IT guys always want the fastest and the newest. But the operation guys always want the most supportable and predictable. VB (at least 3 and 4) may not be the fastest and most efficient, but it has a pretty big library outthere. When Xenforo has a library that covers as much as VB, it will be time to switch for the performance gain. I think the then new owner of VB did the right move back then. The ability of VB4 to take advantage of the existing VB3 library is much more practical, and gave VB3 owner more incentive to update to VB4. I am not sure VB5 can continue to leverage this advantage if it cannot have at least decent enough performance.
Nobody is making addons for vB5. vBulletin is going the way of the Dodo bird. Digitalpoint.com itself switched to Xenforo for good reasons. So should everyone else.
AND A YEAR LATER from when this thread started, I also have DUMPED VBULLETIN and I am now using Xenforo
Xenforo all the way in , i have used vbulletin but xenforo is damm to amazing skins , and addons are great. However everyone has different opinions about that, it's like a question you would never get a truly answer.
Check this out: http://en.wikipedia.org/wiki/XenForo "[XenForo] is developed by former vBulletin lead developers Kier Darby and Mike Sullivan." That tells me enough.