In terms of the conversion, you can only convert 1.x versions of IPB to wtcBB. So if you're running 2.x, its out of the question. And even if you were running a 1.x copy, I don't believe (never checked though) that IMPEX does wtcBB to vB conversions. If you are desperate to switch in the meantime, something like
SMF might do, as it supports conversion from IPB (though is not tested with 2.1.x), and vB supports conversion from SMF.