Impex supports IPB 3 but not the newest versions.. I've lost track but you could ask in vb presales. If anyone is thinking about using Impex to go from ipb to xf, I would encourage them to not upgrade ipb to the newest versions which Impex may not support.
Any version of vb you want to buy that has download access would suffice. The version you need would be vb3.7 or vb3.8...
Alternatively Floris over at xenfans performs conversion services, I believe he does IPB.
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Every 250,000 additional posts is $10.
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I have converted my IPB 3.1.2 site to xF by way of Impex. I know Floris does this for a small fee, which I think is reasonable, so I would agree with the others, if you don't have vB or have a friend with vB that could do it for you, then I would go to Floris vs buying vB.
after contacting kier about ipb conversion he didnt deny it wouldnt be produced so i would say dont buy additonal service of conversion when you can get something for free. Wait till stable release is out and see if import is supporting ipb