The electric vacuum pump under the rear seat is for the door locks. Possibly for the orthopedic seats if you have that option.

Engine vacuum is used for everything else including the brakes, emission controls and ACC (maybe that's everything else). There is a check valve to isolate ACC leaks from the engine and an ACC vacuum reservoir (so to speak) to tide you over in low engine vacuum conditions.

I don't know if the rear headrests use vacuum and if so which source they use.

Sixto
95 S420
87 300SDL