Incidentally, this never worked. The DVD-A help suggests using the “order titles” function which doesn’t solve the problem.
After driving myself mostly insane, I went back to Vegas, edited all the clips together into one big movie with markers. Then in DVD-A I imported the movie for as many times as there were clips, using the IN/OUT markers for each clip area.
Then I added empty buttons to a menu that pointed to the appropriate movie section. Voila, the next chapter buttons on the remote now work.
Shame I couldn’t use my separate clips, but the solution worked and I was able to get the DVDs burned on time.