OK, good show.

I'll second the suggestion by mplafleur that you remove the right side of console, unplug #3 vacuum line, put a tester (MityVac, etc.) and see if the operator holds vacuum. An intermittent vacuum leak in the dashpot or line could do what you're describing, and if too much vacuum leaks, the defrost vents will stay open in default. There is supposed to be a restrictor in this line, so an open line may allow some residual vacuum for the other actuators.

Of course, an intermittent electrical connection to the #3 vacuum switch could also be to blame, to put the vacuum gage on the #3 pipe on the manifold to make sure you're getting vacuum.

Hmm, if you have some extra vacuum line, a rubber fitting and a rubber "T" connector, you can leave the vacuum gage connected while you drive it. If you're lucky it will malfunction while you watch. At that point, you can see if you are losing vacuum faster than #3 pipe can supply it, or if the switch has closed.

In any case, good hunting, and keep us posted, please.

Best Regards.
Jim