I recently had this issue in my '86. Vacuum source can be taken from the intake (for my newly "swapped" powertrain). Do as previously posted. There's a vacuum line that passes through the firewall via grommet near the heater core. If the line (very small) is old/brittle, be careful. There may no be enough of a seal joints where the line attaches to the rubber sleeve.
I had nothing but lower vent blowing heat at my feet. Fixed that connection, and I have full vent functionality. Now... I just have to fix my A/C.
