One quick way to check for vacuum issues is to see if the system responds with air out the proper vents when you ask it to:
1. Hit the recirculate button and see if you hear the door pop open behind the glove box, also, the air should get louder.
Also, if you have good residual vacuum, you should be able to turn the key on, engine off, after sitting several hours, and hear the recirc door move. You can also see it, through the little panel in the back of the glove box.
2. Turn the temp down to cold, the air should switch to the center vents.
3. Turn the temp full hot, air should switch to floor and side-dash vents, with a little from defrost.
4. Defrost is the default, so if it's on defrost all the time, could be vacuum.
Turn it on defrost mode, and it should definitely blow hard out the defrost vents. turn the temp all the way up, it it stays cold, it's probably not getting hot water through the monovalve.

A quick check of the monovalve: warm the engine, turn the system on and adjust the temp dial to the approximate ambient temp inside the car (say 70 degrees). The fan should slow down, meaning it's detecting that it's close to desired temp inside the cabin. this should put the monovalve in cycle mode.
Go under the hood and put your finger on the top of the monovalve, and feel for a click every 5 seconds. If you feel that, it is probably working (maybe).

Just a couple of things to try; these sytems can be pretty complicated; but it seems the mV, the vacuum dashpots, or the control head solder jointsa are the common bad guys.

If you fix it, then you can help me find my intermittant vent issue - works great, then suddenly switches to defrost without warning. GRRRRR.

DG