Well I have been admin on phpBB, IPB (invision), vBB and SMF (Simple Machines) and I thought to share my opinion.
All this "vBB is faster" is nothing but bull****, take a look at http://forums.gentoo.org/ it's huge forum powered by phpBB with great load times.
vBB is great for "setup and forget" as it's really stable and you get support in case something happens. Hacking vBB is not as easy as IPB, but it can be done with some ammount of trouble.
phpBB as mentioned before is really simple and powerfull tool with large community. It's only forum soft that has support for other SQL servers besides mySQL which makes it unique (IPB can support postgre too but not out of box).
SMF is new forum soft based off YaBB, but totally rewritten. It's still beta, but I will keep my eye on it as it's seems promising.
IPB is wonderful FREE software with tons of features. New version 2.0 should be stable soon and it will have almost (if not all) features that vBB has + some other unique ones. IPB is really easy to hack and you have LARGE database of mods for it. Myself I prefer ipb specially once 2.0 stable is out.
As for free skins...I haven't seen much if any for vBB. Due to beta stage there aren't any for SMF, but there are tons for phpBB and IPB.