Croweter, the vacuum distributor locates just above the the fan switch, it has a climate control mode slider to the front panel which allows you to select, Air/Vent...etc
There should be vacuum hose routes from the engine to the vacuum distributor (also known as Vacuum Selector) labels S (source), check that you have sufficient vacuum at the end of the hose. If you do then problem most probably lies in the distributor (assuming the diaphrams and hoses are in good nick). You will need to remove it and change the seals and/or apply some grease or swap it over with another one, it's pretty self explainatory once you disassemble it.
Heres a pic of the layout.