I have no idea how large of an MPEG-2 bit rate file Vegas will accept before it starts choking. On my previous computer, a very high bit rate file choked it completely. The new one handles them much better.
All I can suggest is to try a test with a few different settings and decide what works best for you.
If it was me, I’d render to AVI. Vegas handles that file type easily and the quality should, in theory, be better than MPEG-2.