I've always been curious. Who is it suggested to render separate audio and video files for DVDA? And why can't you use just a standard MPG file? And who AC3 as the audio format? And, (this is the most puzzling), since I can render in AC3.....why can't Vegas read/play it back???
In the US, an MPEG audio track is not part of the standard. Only AC3 and PCM. DVDA will have to demux the mpg file before it can create the DVD so why not give it two separate files? You have to remember that a DVD can have multiple audio tracks so all of the tracks (video and audio) are treated as separate entities.
Originally AC3 was only an audio delivery format, it was with recent video cameras that attempt to make a DVD ready file that AC3 became a source format. The original design of Vegas did not have a reason to edit AC3.
Great answer, David!