If you render in Vegas to Main Concept DVDA compliant MPEG2 video stream and audio to AC3 then DVDA should not have to rerender it. For a 70 min video you should be able to use 8,000 000 CBR setting but its always good to check with a bit rate calculator when you start to get over an hour.
DVDA will rerender if your video is too long to fit, or if it isn’t DVDA compliant. This is the way to keep the best quality.