Okay, I know this is an awkward thing to make a first post about, but I'm not only good with Pokemon, I am good with web development, and I know one good thing about vBulletin 4: it can look nice, but as a whole, the entire product is a pile of junk.
You see, Jelsoft, original developers of vB, got acquired by Internet Brands. None of the original developers work on it anymore, and thus, it has morphed into a bloated junk heap.
Firstly, from what I've seen, the theme system is a total nightmare. You can't just use normal CSS, you have to sift through this idiotic "stylevar" system of variables accompanied sometimes by CSS-like options. Ugh. It's a little harder to make a theme that isn't just a recolor of the default.
Even worse, I heard the CMS system for it, is also not very good either, and isn't worth it. The fact that there's an entire blog about how much vB 4.x is bad, truly shows that it is not very good.
From what I've seen, IPB 3.1 actually looks pretty good. They got the social stuff going as good (if not better than) vB 4, the profile pages look pretty good, there's Facebook/Twitter/OpenID support out of the box, they got a nice extension system, and yes vB 3.x can be converted over to it too.
While a forum upgrade is a good maitenance task, you still have to remember that you have to keep the end-users happy too. A forum I used to go on a lot upgraded from 3.6 to 4 recently, and a lot of people hated it so much that they ended up going on a somewhat related forum to escape it (it's hard to explain without naming names, but there were also some tensions with the new staff at that point, and the upgrade removed some custom features, but I swear the other site had a slight activity increase post-upgrade. The other site btw, runs the same version we got now)
tl;dr vB 4 is a Feebas. We want a Gyarados.