It is your monovalve. There is no way to get hot air if the monovalve is closed. And there is no way to get cool air if the monovalve is open.
The vacuum system has nothing to do with temperature. Just with direction to the different air vents.
If you get hot air in the center vent, it is due that the vacuum system ignores that there is a problem with the monovalve and assumes there is no hot water in the heater radiator and tries to drop the cabin temp asking the fan for more and more speed (in "Auto" mode), opening the center vent that is designed to deliver only cool air (but the heater radiator has hot water, meaning hot air).
Next week i will post almost everything about monovalve, pictures and how to service it (yes, you can let them like new for less than $ 30. Besides, a little description how the heater system works. Meanwhile, disconnect the monovalve and apply 12 volts directly from the batterie (polarity is not important). You must hear and feel a notorius click.
If not, i'm shure you will not, try to give it a couple of knocks to release the piston inside the selenoid (the monovalve is a selenoid), fixed with the coil together due rust. Of course, if you get only cool air, the problem may be the same. A fixed piston inside the monovalve. Be patient. Next week i will post something can help you.