[yondering]

Thanks for the reply.

I've been investigating this problem further since I posted, with the same idea as your suggestion. The problem does not seem to occur during any low vacuum conditions, necessarily. When it did occur, I tried downshifting, and using engine braking to supply maximum vacuum to the system; this did not cause the vents to come back on.

I did discover that when the top vents stop blowing, the air comes out the defroster, rather than being completely blocked off as I originally thought.

I think I've identified the problem as the little electric/vaccuum control unit mounted to the blower housing below the glove box. I removed the kick panel and drove around until the vents stopped working. At that point, I reached down and tapped that box with a wrench, and the vents started working again immediately. I suspect the problem may be a bad electrical connection in the box, as I can't find any vacuum leaks around it. Unfortunately, it looks like you have to pull the dash to get the mounting screws out of this little box.