The main reason people used to buy vbulletin not because of it was solid, fast and non-buggy. It was the existing of vbuleltin.org, plenty of plugins, templates, for free, to try things and to learn from a friendly community.
Well, nowaday these things are no where to be found
vBulletin.org is dead, so does Vbulletin 5 (at least).
Behind Xenforo success is sadly a strong push from vbulletin itself.
People disapointed with the latest release of vbulletin 5, since they were patiently waited like years to have a STABLE release of vbuleltin 4, they expected something outstanding, but we all see what they got, buggy, slow as hell vbulletin 5 and most of the well-know developers never recommend it, well actually they do but it was highly recommend people to AVOID it.
If Xenforo was another good forum software, it would never reach the position it is right now, but since the PR was good, "The former lead developer of vbulletin 4, the developer of vbulletin before it goes mad", people really buy that.