One "negative" thing with vBulletin is that it's a pretty resource hungry forum, and needs a good (dedicated) server in order to run smoothly.
Many shared webhosting companies doesn't allow vBulletin to be installed on their servers, since it may slow them down...
There's lots of pros and cons with both boards
phpBB is free, but vBulletin costs you a pretty penny, so it may not be a good idea to use it unless you're sure that your forum will be a "success"..
There's been more security issues with phpBB than with vBulletin (but a few have been discovered on vBulletin lately)
Most of the mods on phpBB is free, and it has a great modding community.
vBulletin do also have many great mods, but many of them are not free.
vBulletin is a (much) more moderator/admin friendly board, and you'll need to mod phpBB (a lot) in order to get close to vBulletins features.
The biggest forum on the net uses phpBB by the way..
www.big-boards.com