I suggest you enter the search term - Export Vegas EDL- for last 6 months of the VegasPro forum. This topic has been covered in many ways..by doing some research here and the references made throughout this forum's discussion - you may get some clues.
I do some post audio in Vegas. (and PTs) Prior to FCP-X, an OMF could be exported form FCP, though the OMF carried little video info, and a third-party OMF converter was needed. AAF is a popular cross-app. conversion utility. However FCP never supported AAF (w.o AutoDuck) and FCP-X doesn't either, and AD is not an option... Even if it was, Vegas' importing of AAF files from other app.s, I would not count on.
There's been other discussion on this however, so perhaps search this forum.
Many have tried... few have succeeded (maybe none).
> "Is it a nightmare?"
Do you enjoy "Night of the Living Dead"? because the problems keep coming, and coming, and coming like an endless stream of zombies. ;-)
Think about it for a moment. Transferring an edit from one program to another is hard enough, but going from Mac to PC is next to impossible. Unless this is a "cuts-only" edit, which I doubt it is, the Video FX available on the Mac don't exist on the PC, the audio FX available on the Mac aren't available on the PC. You would loose all of that and probably more because of any build-in capabilities (cropping, re-timing, etc.) that FCP has that Vegas doesn't or does differently. It really isn't practical to expect this to work cross-platform.
> "I have a potential client who has a project on FCP and may want me to change the edit."
The question is what do they expect back? Just the final footage? or do they expect an edited FCP project so that they can give this to the next editor they hire to make future changes (like they did with you)?
I would recommend that you buy a Mac and FCP if you plan to work with FCP customers. Otherwise I wouldn't accept jobs like this. Being on a PC with Vegas isn't going to cut it due to the many incompatibilities.