This is comparing to PHPBB from view point of someone who runs one at home and admins a vB at work.
1. More admin options
2. Easier admin interface
3. Easier mod interface
4. Faster
5. More features out of the box
6. Better documentation
7. Better permissions system
8. Easier to modify. And this is coming from a complete PHP beginner
But it does cost money and it's worth every penny of it. I've considered pirating vB for my house, since I can't afford to shell out the dough, but given the product quality for the price I just couldn't do it, which is why I'm running PHPBB at home.
At the end it's your call, you can either save up for vB or get one of the free ones.