Not to Ruffle any feathers, but VRO operate off of crank case pressure period.
The warning circuit uses tach inputs to verify sufficient oil is being pumped as required per engine RPM's. So if your tach circuit is shot it doesn't mean the VRO isn't functioning, it just means you won't know when it does until it's to late.
From
http://continuouswave.com/whaler/reference/VRO.html
The VRO's electronic circuitry compares the RPM of the engine to the number of pressure pulses from the oil pump piston. As the oil flows into the fuel area, the pressure moves a small piston that actuates a steel pin in the alarm module. The pin's movement is detected electronically and "counted." If the engine revolutions exceed the preprogrammed oil pulse ratios, the warning horn is activated sounding a rapid on and off sequence.
I'm with Phester. VRO's rock! but they do need to be maintained like anything on a boat. Unfortunately they are expensive and they aren't very well understood which only adds to the topic raising its head from time to time.
Kamikaze