The original 30 kts/300 fpm was for when helicopters were very light in weight and had very little downwash, but nowadays I believe the CAA would expect to see 400 ft per minute. It is in fact a safety margin - in reality you would need to be inside ETL (around 12 knots) where the downwash stays more or less within the rotor disc.
The figures for rate of descent will vary with the aircraft, but the heavier it is, the less likely you will get into VR because you will need to go down faster to catch up with the downwash.
Never heard of a warning light for VR..... Is that like the red light that comes on when the engine falls off a DC 10?
Phil