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?