if you have an already finished M2T file from Vegas, I'm assuming you're not asking for rendering settings? or did I misunderstand the question?
it sounds like you're saying that FCP can't take an MPEG2 file, and you need to transcode your file into something that it can handle ( perhaps this should be asked on a FCP forum since MPEG2 files are fairly common? )
MPEG 2 is lossy, so to get the best possible quality, I'd recommend exporting as a Quicktime movie and choose a codec ("Video format" in Vegas' render box) such as Component Video or
PNG.
There is a new tool that works fab for converting m2ts of all varieties to .mov. Before you had to transcode to prorez or quicktime with a generational loss and it took forever to transcode. This new app is called ClipWrap and you just drag and drop and it changes the wrapper from mt2 to .mov perfectly size framerate & timecode intact and it is as fast as a file copy.
It is mac only. So if you dont have a mac you can just have the fcp recipient convert it on their machine. I wish I had this last month when I was trying to covert 14 hrs of Vegas M2Ts to FCP
There are several good options:
1. Uncompressed Quicktime MOV. Will create huge files. Lossless.
2. Cineform AVI (they offer a free decoder for Mac). Visually lossless.
3. Huffyuv AVI (needs installation on both Windows and Mac). Lossless.
4. Avid DNxHD. The encoder is free on both platforms. Visually lossless.
I suggest you try Cineform, and if this doesn't work, try Huffyuv.