Vents

Under the hood on the passenger side of you engine compartment, you should see a black plastic "ball" near where the A/C hoses run into the fire wall. The ball should have two vacuum lines running to it. Check the the larger of the two lines for vacuum while the engine is running. Make sure that the smaller line isn't broken as it runs into the firewall near where the passenger side fender meets the door. If all of this is OK. Check for a vacuum leak at one of the hoses under the dash where the line that runs from the ball connects to the heater/a/c "box" under the passengers' side dashboard. Repair any leaks found. Keep me posted on your progress.

Under the dash on the passengers' side, the vacuum lines run into a connection which has many vacuum lines (multi-colored, they kinda resemble wires). These vacuum lines activate vacuum actuators which will switch the air from defrost to floor and then to vents as you move the selector.
If all of the lines are getting vacuum when you select either floor, defrost or vent, then the suspect part is the switch itself.