Personally, I wouldn't ever pay the steep pricetag for something that is free. I think phpbb is better anyhow, so it definately kills Vbulletin.
I love vBulletin but a friend bought it and we use it. I love all the smart guys with their add-ons and everything. Mine is with Gambling. I love it.
Is there a reason to change and what will you loose if you change? If you installed lots of mods, changed a lot yourself,... then perhaps it's just better to stay with phpBB
vB is by far the most professional board with the (imo) best hacking community. As free alternative I'd also go for SMF or check out the new phpBB3 Beta.
phpbb and vb are vast differences its like comparing oranges and apples. If your community is based towards on topic it could be better to use phpbb cause you dont need all the features etc. If your looking for features and have a community that knows how to use a website properly then they will like vb. I personally use both i have a mybb forum but the mods are eh not enough so i will change it to phpbb probably maybe vb. Both are good choices But when I consider either phpbb or vb I dont consider the price but the needs of the community