Render the video as MPEG-2 using the appropriate DVD Architect template. Render the audio using the AC-3 option, and the Stereo or 5.1 template. Use the same file name for both (the extensions will of course be different). Import into DVDA.
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When will Sony change Vegas to:

1. Avoid having to render in two steps?
2. Provide a "fit to disc" feature in Vegas? (Obviously will not work in all cases, but would be extremely useful to many people.)
3. Provide a big fat button that says "press here to put your video into DVD Architect"?

It is obvious that a LOT of people find the workflow between Vegas and DVDA extremely unintuitive and awkward. I bet almost every new user has had problems figuring this out. And, even after you figure it out, it is still a kludge.