I personally have three different vBulletin licenses, one owned and the other two leased. I am a vBulletin fan all the way since vB 1.3. I actually still use vB2.x on one of my sites which I'll need to upgrade soon.
As for open-source, both phpBB and SMF are great choices. phpBB was really popular over a year ago before literally big flaws made it impossible for some service providers auto-suspending all accounts running phpBB until they released a patch/fix for the flaws. However, I believe phpBB is secure as ever nowadays as I don't see many forums getting hacked as often as it were a year ago. There is a very big backing of phpBB still; if I did not have any funds for a forum, I'd personally choose phpBB.
SMF is a popular choice too now but definitely a newcomer in comparison to vBulletin. I'd say give both a try and see which one works best for you and your friends.
