Nick,
I gave an example of a helicopter that can experience VRS at above 35 knots.
One reason books explain it in this manner is that we don't use sophisticated equipment to determine airspeed or ROD. Vertical speed indicators and airspeed indicators are subject to many many errors. Therefore, in consideration of safety, most books overstate the regime in which you can experience VRS.
I have experienced VRS in an R-22 at a ROD indicated of 400-500 FPM, but in theory, I should experience VRS until I achieve a ROD of over 900 FPM.
So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.