Each one of the vacuum lines connects to a vacuum "motor". When vacuum is applied the motor is in one position, and without vacuum it is in the other position. The motor (actuator) is connected to a "door" in the air ducts that directs the flow of air to the desired vent ports.
There are individual actuator doors for the defrost vents, dash vents, and floor vents. There is also a door to block or permit the flow of fresh air into the vehicle.
The hoses connected to the switch will travel behind the dashboard to the required vacuum motor. If you can access the end of the hose as it connects to the motor, you should be able to disconnect it, place your finger over the end and feel the suction. The vacuum pressure and flow should both be "strong", keeping in mind the small inside diameter of the vacuum lines. Using this approach will help to determine if the motor is faulty, or if the vacuum lines/controls are not providing the proper air flow.
Also, whenever you have a vacuum leak, it will usually result in a rough running engine and/or power train codes. The excess air entering the intake causes a lean mixture, which the PCM detects. A very, very light leak (one that probably wouldn't cause any problem with the operation of the heating system) may not cause a CEL or detectable engine misfire, but still could be noticed using a scan tool, showing low short term fuel trim readings.
If you believe you have a vacuum leak, the easiest way to find it is to listen for the hissing sound. Since most of the vacuum lines are located behind the enigne, this task is more easily performed with the cowling removed. I have found several damaged vacuum lines using this technique.