The vacuum source is that little, tiny black hose...you obviously know which one. Either that hose or the elbow can be bad without showing signs of cracking...or they can actually be blocked. Check the actual pass-thru at the firewall as well. From there, the next easiest thing to check is the vacuum canisters under the dash. One on each side, accessible by removing the black, carpet covers under the dash. You'll see them right away, all the way up there under the dash cover. They shift the air flow....or actually block the airflow to the floor vents, forcing it out the dash vents. If, when you change the pushbutton setting on the dash, either of them don't work, you have either a broken hose or a bad diaphram in one of the canisters. If you don't get vacuum at either one, trace the hoses back to the pushbuttons. I've never, in twenty years, had one fail. However I have had hoses come off behind the button unit.

If all hoses are intact, check the white, plastic vacuum canister behind the console. That's where your engine compartment hose goes to. I did find that one had been broken by some sort of impact once years ago.

DS