You could use ffmpeg for those things if you’re alright with a bit of command line. Getting audio would be “ffmpeg -i input.mkv -vn -c:a aac -ac 6 -b:a 640k output.m4a” and getting video would be “ffmpeg -i input.mkv -an -c:v copy output.mp4”, I think. I don’t know how Vegas would take those files, but they work well for me in Premiere.
Are you intending to only render it out in stereo rather than 5.1?
Sade, just resurrecting this to say thanks very much for this. I’ve now got to grips with ffmpeg, and am demuxing audio and video now with a single combined command, which is super easy now. I’ll also be editing and outputting in 5.1 surround. This was a really helpful direction, thank you.