I've used phpBB for a number of years and look after two Vb sites, the main reason for the phpBB is because it's free, but as with everything you get what you pay for. If you really want to manage your forum and keep a watchfull eye on it 24/7 then phpBB or any of the free forums is going to be ideal. I've had far less hassle with the Vb forums, simply because they don't go wrong, and moderators and admins on those forums can be trusted enough not to wipe the thing off, where as phpBB it does give you the ability to self destruct the forum.
A few years ago we had one forums account locked down by the host, as some moderator was playing with the word filtering system and included the word 'the' in a filter phrase - well you can imagine what happened!
Vb is brilliant, if you're going to try and cover your set up costs / hosting costs, at over £200 for a brand free package along with some extras it's costly to those who just want a forum for a community without any cost recovering excersises.
(cheaper to buy Vb in dollars by the way).
My opinion is phpBB is for those who keep things up to date and monitor them heavily and wish to pay nothing but can expect spam or a hack at some point, and Vb for a ease of use, installation, support back up and to know that you've paid for a brand for reason, because you'll be secure-ish.
If you have the pennies spare - Vb - if not - save up! It's worth it!