phpBB would get my vote.
1) It has tons of contributors, improving it and finding bugs (as with any open-source project)
2) There are tons of user-uploaded mods, hacks and styles
3) I've found phpBB's code to be quicker to hack/modify as well as customize. I set up one from scratch, crated my design style, customized access lists, etc.....every option of their Admin...all in about 1 hour.
I've seen phpBB servers with 10,000s of users and obviously vBulletin here has way more than that.
Run the latest MySQL and you shouldnt have performance issues unless your server is really outdated or misconfigured.
