Disclaimer: I use phpBB on my site. I've never used vBulletin.
I just took a look at the sites for both vBulletin (http://www.vBulletin.com) and phpBB (http://www.phpbb.com), and I'm not seeing much difference when it comes to features.
In looks like vBulletin (http://www.vbulletin.com/features/) has a calendar and an administrative statistics page. I couldn't find that on the features page, I had to go in the admin demo to see this. In phpBB (http://www.phpbb.com/features.php) you get a much bigger community making mods and it's free to use and upgrade forever.
So vBulletin seems to have a few more features, but you pay for it. I can't comment on the robustness of it. My wife is a member of communities of both types and she hasn't noticed much difference in them. So I guess it's the old trade off of commercial vs. free software. Do you want to pay for commercial support or not?
Anybody else have anything to add?