Thank you, Edward,
for your succinct but clear guidance.
I can render the two (original plus narrative) in Vegas and create a new combined audio and render it.
In DVDA 3, it has a feature: “Add Audio Track” (This adds another track below the existing one. I assume that’s what you are referring to? And this is where the rendered, combined audio track is inserted?)
In DVDA 3, you mention using the Menu for the viewers to toggle between one audio track or the other (combined) one. Does this imply that, in creating the DVD, I must use the “Menu Based” method, (or can I still use the “Single Movie” method?)
John A