phpBB is simply too popular and too easily spammed if you don't have the time/patience to wade through it's documentation and mod it to a decent degree.

I threw up a domain for a friend's band recently, about 3 months ago. They installed phpBB, and within a month the spam-bots had completely taken over auto-registering and auto-posting on the board, to the point of 10-15 new users per hour on a VERY new domain. I've modded phpBB in the past and while there are indeed some decent add-ons out there for it, it's mod system and communities are nothing like vB's.

My advice is that unless you can devote a decent amount of time to it making it spam-proof, shy away from PHPBB.