For the most part, vacuum circuits in the Heater/A/C units are deadheaded, providing there is not a leaky selection valve, broken tubing, or a broken vacuum bellows for the function the motor is controlling.
You might put on a hand held vacuum pump onto the hose going into the firewall and listen for a flap to close to make sure it works and does hold vacuum.
But it would seem to me if one has heat wih the vacuum line hooked up, then one would need to leave it connected.
But, it is strange vacuum is needed to get heat. Usually, it 'defaults' to heat, it would need vacuum to get A/C and to route the air flow to feet, dash, or defrost.
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