Vacuum loss affecting AC vents?

There is a check valve in the headlight vacuum line which goes to the headlight system, check this valve.  It is covered at the link that Aaron posted.  check the vacuum lines where they connect to the two headlight vacuum relays, hoses tend to loose their elasticity with age and may not seal well there.  when checking the vacuum can, check the upper end where water can collect and rust out the seams, mine had lots of small leaks where I couldnt see them.    There is a plastic "T" up near the brake booster that sends one line through the firewall to the heater controls, check this "T", mine was cracked just enough to keep my vacuum actuators from working the air control doors.  I believe there is a vacuum line which goes to the gasoline vapor recovery (charcoal) canister. All of these lines which leak may add up and steal the vacuum you need to operate other "stuff".