New heater core & valve- still no heat

I have a 99 Tahoe 5.8 vortec with 78k.I changed changed out the heater core and heater control valve but still no heat. I noticed that the vacuum was always on regardless of the temp setting. When I disconnect the vacuum I get heat. Question 1. What would cause this? 2. if I leave the line unplugged (It's been cold here) what else would be impacted?

thanks in advance for the feedback.

I have the same vehicle and from what I have read in the shop manual, the only thing that vacuum controls is the AC. The Temperature (hot to cold) dash knob controls an electrical actuator down on the heater box that adjusts s damper hot to cold.

This may not help, but it's what I have figured out so far.

Take a look under the pass side dash and look for a cable that opens closes an actuator. I've seen the snap-lock type knob that holds the end of the cable break off before.

Just curious, but what vaccum are you talking about. Location?? When I looked it up everything that controls heat is electronicly controled. My only guess is maybe vaccum system that when the engine is cold shuts off the heater control valve to get the engine to op temp the fastest. I am looking for anything like that and can't seem to find it, but you never know