Yes, Defrost vent only means no vacuum for vent selector, defrost is default position.
I think there is a Black vacuum line and a grey vacuum line that run thru the firewall into the cab.
Black line would be the main vacuum source for Vent selector, and also Heater hose bypass, the Grey vacuum line that comes out of firewall and goes to bypass valve.
Black line from firewall would connect to Vacuum reservoir
Red vacuum line that comes from intake manifold is the vacuum source it also connects to vacuum reservoir, it can change to a Black color line along the way.
So you have this
Engine/intake manifold vacuum========Vacuum reservoir=========(Vent select====vent select actuator) () = in cab
Actuator is behind glove box, video here: www.youtube.com/watch?v=7tSFPSY19MI
Find correct red vacuum line on intake manifold, there can be more than 1 red one, never assume :)
Pull it off, pull off the other end of this hose from reservoir
Suck on one end with finger over the other end, if it holds vacuum then it is good
Reconnect to reservoir
Unplug other line on reservoir should be the Black line going to firewall
Suck on the red line again, with finger over open port on reservoir, should hold vacuum, if not the reservoir is cracked, replace it.
Now with Black line disconnected from reservoir, suck on it, make sure Grey line is plugged back in to Heater by-pass.
It should hold a vacuum, if not get someone inside the cab and while you BLOW into the Black line have them listen for air leaks