The proper term for the vacuum line that feeds the HVAC system is accessory vacuum. It looks like you did not look at the right vacuum diagram. See pic below.
The symptoms you will get with that specific line disconneced is improper vent control and airflow. The bottom line is it is needed and must be connected since the HVAC system uses vacuum to control the various actuators under the dash. The line enters the firewall and feeds into a vacuum distribution block by the blower motor assembly. One symptom is you will have no control over the center dash vents. Air will flow out the defrost and floor vents only.