A plug is anything to stick into the hole. could be a screw or bolt or they make plastic plugs that are sized for the small dia of vacuum hoses

At the rate we're going here it will be winter and the defrosters will be needed.

you might be able to cut threads into the busted cruise controller servo to allow you to thread into it a brass pipe thread nipple, along with some sealer on the threads, then to that atrach your vacuum hose with an apropriate sized rubber nipple adaptor. stick with a small diameter thread size, 1/8 pipe thread or less if possible. you want a fine thread as I assume the servo body is thin, fine threads giving more threads to bite into the thin wall of the servo.

that is how Id try to repair that servo fitting I think