[randy500]

Isn't it clear from the vacuum diagram that there is one vacuum switch that when activated does the following:
1. Shuts off the heater hot water flow to the heater core
2. Opens the vent on the passenger side
3. Closes the cowl vent

All of that happens at once, the cowl closes to allow air to be recirculated from the inside passenger vent that has now opened giving recirculated air to be cooled by the evaporator.