What is magic about 2500 fpm rate of descent....most autorotations can provide that ROD....it is staying in the downward moving column of air that is being accelerated by the rotor system that trips you up. 300feet AGL and 60 Knots used to be our approach gate in the Chinook with the Thrust Lever full down....then a transition to full power getting the old girl stopped at a hover. It idd not matter if it was a 105mm Howitzer and A-22 bag of ammo or an internal load. The object of the exercise was to spend as little time between 2500 feet AGL and the Landing site. I would suggest airspeed and power setting is far more critical to the onset of VRS than Rate of Descent. One does not turn into a Frog at 800fpm ROD and 40Kts IAS.....there is far more to it.