I've been using phpBB for 9 years now. It's first version or two were kind of behind-the-times. But the current version is really great.
I have used vB before but found it hard to figure out how to update the template to give it my look and feel, plus installing updates seemed difficult.
My vote is phpBB because I have been using it for a long time and I find it easy to update the templates, easy to update when patches come out, etc. The fact that it's free is a bonus ;-)