If the car is a VS Exec or similar with manual heating/air con then behind the switch that changes modes is a vacuum switch which connects all heater and a/c vac lines into one. When the switch poops it self it will change the heater direction whilst accelerating. New switch and probs gone. Need to pull some dash trims off to replace.
When you hooked the Vacuum up did you notice a small valve in the line, This valve is a 1 way valve and if you blow through it the end that you can blow through pionts away from the heater vacuum hose. I just done a conversion and put this valve in backwards and the heater only blew on my feet, turned her around and bingo easiest fix of the decade.
The hose has a T in it 1 of the hoses goes to the Auto Dip stick (Ithink) and the other goes to the heater. It's in the engine bay at the back of the motor in the middle. The main vacuum hose seems to come from the back left of the engine block. The valve is white on 1 side and black on the other.