SSD
The venturi is indeed independent of the engine but not independent of airspeed! We have a vac gauge and it drops out of the green at anything less than normal cruise. It's also temperature dependent, you get less vacuum on a hot day than you do on a cold one.
Reading this thread stirred a memory. I used to share a Fairey Swordsman powerboat with a friend. When we rebuilt the panel we put two dirty great red lights on it. The idea was to fit a tee piece into the oil pressure gauge take-off on each engine and plumb in a pressure switch. A dirty great red light is more attention grabbing than a drop in pressure on a gauge.
I sold my share to my partner before this was completed. The inevitable happened. A year or two later he had still not finished it, an oil hose burst and an engine seized because he hadn't spotted the gauge dropping to zero.
We have low voltage warning lights but we still don't seem to have progressed to providing them for loss of oil pressure (or vaccum for that matter). And if you'd like to fit that simple safety device costing but a few pounds to your aircraft sir that will be loadsamoney please.
Mike