I would suggest airspeed and power setting is far more critical to the onset of VRS than Rate of Descent.
Sas,
Those are of course the other two critical elements in the establishment of VRS that go without saying. It has been discussed ad infinitum on this thread that in the Marana situation the aircraft was slowing rapidly with a tailwind and max power was being pulled in at 285ft agl with a 2500+fpm ROD putting it in an unrecoverable condition whether it be a tiltrotor or a helicopter -- at that point both of them would turn into a 'Frog.'
21stC