Look into SMF, simple machine forum. It is a free, full featured simple forum and is easily expandable. It is very popular right now and being very actively developed.
http://www.simplemachines.org/
To me it comes down to this:
If you want it free, go SMF
If you want the best, go vB
phpBB and Invision aren't moving to keep up with the times, vB and SMF can.