If the system is not holding vacuum when the Key is on, then you will need to remove the Console Side panel and start testing the various vacuum circuits. From your description one of them is not holding, most likely a failed vacuum modulator, could be a failed solenoid however less common.

You did verify the presents of the outside temp sensor, as an open circuit on that or the inside temp sensor will also give you full heat.

Take you tester and see if the Heater control valve holds vacuum, directly connect to it ;)

The vacuum modulator gets manifold vacuum, a separate system from the HVAC and Cruise control units.

Otherwise exactly the right directions -- look for the failed actuator in the HVAC system. The WSM has a pretty good "truth table" for troubleshooting some of the vacuum actuators as groups. The heater control valve is under the lower half of the aircleaner housing on the right (passenger) side of the engine. Check that first since it's the easiest. Then through the left (driver's) side of the console to access the solenoids and the lines to them with your hand pump.

And I make three of us.