This might be better asked on the FCP forum since FCP is very restricted in what formats it can edit. Quicktime 7.5 for Windows has added the ProRes422 codec so you will be able to edit the files from FCP in Vegas but, unfortunately, Apple doesn’t allow you to encode using that codec (it’s read-only) so your challenge is what to render your files to so that FCP can read them. That’s why I say ask on the FCP forum.
Maybe the CineForm codec would work since they have NeoHD for both the PC and Mac?
~jr
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