Like I said earlier if there is no vacuum to the heater control valve it defaults to the fully open position so I don' think that is the source of your problem however if no vacuum gets to it when the ac is turned on that in itself is an issue.
I suspect that you may have an issue with the temperature blend door actuator. It is located just off to the right of the gas pedal and a little towards the rear of the vehicle. To access it remove the kick panel that is by your right foot and you should see the actuator, it will be a about a 3 inch square box with a plug in it. On that box you will see a white circular area with a square hole in it, this is the drive for the actual door. Change the temperature from full cold to hot or vice versa and see if the circle rotates, if it does this means that the actuator is working if it doesn't chances are that the actuator is bad however there could be another problem. If it doesn't turn you could also try giving it a little tap with the handle of a screwdriver, sometime they get a little stuck.
If the actuator moves and you still do not get any change in temperature the door inside may be broken.