Thank you, Brian.
It turned out to be a quite simple fix but I would not have tried to find the problem without your analysis and references to the diagram.
Your diagnosis that the system was apparently in default mode due to no vacuum pressure made perfect sense and I was actually relieved to think the fix might not involve a dash tear-down.
The diagram from the website link seems to show an inline-6 but I took another thorough look at anything coming off the manilfold (left and right) on the 129's v-8. I thought I would at least inspect anything and everything up in plain view before trying to undo the right wheelwell to find the vacuum reservoir.
Then I saw it.
A small white vacuum hose, 4 to 6 inches long, coming straight out of the upper/center area of the firewall. I also saw an apparently corresponding black rubber hose coming from behind the head just under the cover.
All I had to do was plug the ends of these two hoses together and the problem was solved. All the vent flaps fly.
I can't believe I missed that disconnected hose during the initial inspection. During my initial inspection, based on a search for a vac res hose, I was only looking around the right front underhood area and the wheelwell. When I finally saw the offending disconnected vacuum line, after reading your post, I was looking on the left manifold/firewall area.
Thanks again for you detailed response
