I will try to relate eWebTalk's experience as we have used IPB and vB

We haven't used phpbb but i will give my personal opinion as the end user:
phpbb ->
Cons: if your forum is a commercial area, phpbb sends a bad message as its overall features and look send an unprofessional message. Especially the message of it being free. Now i've seen successfull boards using this but they tend to be more into the fun theme. Insecure!
Pros: Free!

It also has a huge modification list
IPB (you didn't specify version)
1.3-> Pros: Free! Has a sold modification archive.
Cons->Almost the same as phpbb due to it being free. Sort of insecure. Implementing your own design and coding is a headache..
2+-> Pros: Much more secure than 1.3 and phpbb. Nice overall admin cp.Modification is close to that as of 1.3

Price:

is 185 owned, yearly is 69.95 (This seems a lot lower than the last time i checked them)
Cons: IPS (company that makes 1.3 and 2) are still sort of stuck with the reputation from 1.3, but are coming out. Modifications again are a headace, the overall procedure has improved but headache nonetheless
vB->
Pros: Industry standard, all major boards use it. It is secure, fairly easy to modify, both in terms of design as well as coding. Template changes through the control panel are great, especially the way the tools are presented
Cons: Much smaller free modification archive, but still pretty sufficient. Owned license is 160 UDS, leased license for a year is 85 USD
Overall: go with vB, currently it seems to be an allrounder

IPB 2.x+ is catching up though
Try going through each of their features, check their control panels
