vBulletin is a load of crap.
- bad support
- bad themes
- slow software, owing to the lack of neat coding
- it's a nice little pack of crap
- pricey as fuck
Xenforo is just
- bad
- no themes
- VERY GOOD support though, which is pointless if the software itself is shit
- VERY cheap
IPB
- great themes, and I mean, GREAT
- great support
- excellently coded, with meaningless coder comments in between
- not as cheap as Xenforo but not as pricey as vShittulin either
IMHO, most people who install vB haven't actually bought it and have no idea what they're doing. They just like using what they see that others are using.