My 2 cents:

vB Has more (free) modifications that make the product well usable, it also has a very easy way to integrate it with other systems (what we use at TB, rest of TB uses the vB Account system). I agree that vB5 seems to be a complete failure but I've seen the latest beta's and it's going in the right direction. It's far from complete in my eyes but it'll make it I guess.

IPB is great, but also has it flaws in my eyes.

XenForo is long from a completed product, I wouldn't recommend it, would say use IPB or VB then. It's just not great software (at least yet!).

Also large websites like XDA-developers are using vB, so it's not that disastrous as people say sometimes.

My recommendation: wait until vB5 is more ready, then make the choice between either (a cheaper?) vB4, vB5 or IPB