What does the vacuum heater control valve do ?

The heater control valve either permits or restricts coolant flow through the heater core, depending on whether vacuum is applied to it. It is a normally OPEN valve which closes when vacuum is applied.

When the valve is open, coolant circulates through the heater core and you have heat. When it closes, take a guess..lol.

The heater control valve is in the FEED line to the heater core, so if your question that wasn't asked is, how to hook it to the Hemi then identify the connection the Hemi used to feed it's OEM vehicle heater and use that. The return goes to the other heater hose connection that is not on the water pump.

Yes, the vents hang off the dash.

Your vacuum circuit to get the necessary vacuum signal to the heat and air system so it function is this:

See the vacuum reservoir which is the black can on the fender? First of all you need to keep that or the vents will switch when you climb hills.

That plastic can has two ports on the back. One has a line that should be connected to a manifold vacuum port. The other goes through the firewall into the car. I can actually see it from here. There's actually two lines there..the other runs the heater control valve.

It's not hard to hook up. Hope this helps.