Most platforms have converters, so you would need to look up and see if phpbb has a vb converter.

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Hi! You don't wanna pay for the license anymore, huh?
You can try the built-in converter from phpbb or this converter. Good luck!

instead of going for phpBB have a look at some others as well..phpBB is pretty slow.
Have a look at Vanilla Forums of SMF and there are a whole bunch of others.

I think you should try to converter but its difficult to jump from one to another there might be some problem so i would say you should hire a professional persont to do it in order to make it easier for you.

SMF seems to be the most preferred now. A lot of folks are moving from phpBB to Simple machines..

There is an importer from phpbb to vbulletin once the license is purchased

You can use the English version of Discuz. It's free! Downloads - discuzx-en - English Discuz!X, UCenter, UCHome - Google Project Hosting

I've done it before.

But it's a seriously long process.

Took me around 2 weeks.

.... How come no one has asked the OP why he would even want to convert his forum in the first place !?!?!?! VB all the way mate!!

+1 for Vanilla Forums. They have a porter from vbulletin:
Vanilla Porter 1.8.0 by Todd - Vanilla Community