That is an 'Automatic Air Disconnect' system, and has nothing to do with the vacuum system.
It's usually found on vehicles with onboard air systems, especially fire apparatus, that have air brakes, air ride and air bag systems, etc. When the vehicle is parked for a long period of time, the air pressure tends to leak down and can prevent the vehicle from moving in the case of air brakes, until the onboard compressor can build up enough pressure to cause the brakes to release, causing a delay in response time.
The solution is to supply the vehicle with external air pressure via that connector, just like using 'Shore Power' for the electrical system. Since drivers get in a hurry and forget to unplug this stuff sometimes, manufacturers made it automatic when the ignition is activated and the vehicle started. A 12VDC current is passed to the solenoid on the Air Eject device, causing it to release the air hose fitting and eject itself, propelled by the static air pressure in the line.
If you don't want to use it, cap off the line very tightly and just disconnect and insulate the wires.
