The SM forum software has a massive following and some of the moderators & developers have a virtual cult status!
Loads of activity so I would have a go with this before the paid-for vB forum software.
Incidentally, vB has a list links to sites that have bought and used it software - many of these sites are dead - not because of vB but just because that's what forums are like - so I'd do the freebie version first to see if you and your forum have the legs needed to get off the ground.
I believe that the data on the SMF can be imported to vB but you'd need to check out that key step before investing lots of time in SMF incase you do want to move to vB later on.
good luck, Luke