Ugh, thanks. To me, “burned” unambiguously means rendered, meaning only that portion goes onto the DVD. As you note, that’s what Vegas does with the equivalent of in/out points (the loop region), if you select that option in the render dialog box. What the documentation should have said is that the entire source file gets burned, and that the in/out points only affect navigation through that source file in the final DVD.
Guess I’ve lost a days work and will have to start from scratch. That is, unless there’s some workaround to take the new, shorter video file I re-render from the source file in Vegas (using the same in/out points as in DVDA), import it into my existing DVDA project, and en masse (keeping their relative positions) move or copy the chapter markers associated with the larger source file to the re-rendered clip.