The check valve on the can should keep it from getting sucked backwards...
I did climate control, so we always put a check valve for the hvac unit so the doors wouldn't change modes while driving and the vacuum drops.
The laredo had vacuum operated doors and would default to defrost with no vacuum. It was designed to default to defrost for safety reasons. If the vacuum dropped too low, then it would go into defrost mode. If the hvac doors stopped working, then you could at least clear the fog off the windshield.
The limited had electric doors and ATC, so it wasn't necessary for those...