Vortex Ring State (VRS) is a condition where the rotor ingests it's own vortices (over simplification). During powered flight the airflow moves through the rotor system from above to below. When airflow is moving from below to above the rotor, it’s called autorotation. VRS is entered when the flow is roughly between these two states. So it follows, to enter VRS, the airflow through the rotor needs to approach that of the rotor downwash. The rate of downwash is related to disc loading—I haven’t calculated the induced velocity of the aircraft, but I doubt his speed nears Vi. And more importantly, the movement of air needs to be nearly vertical through the rotor system. How far up is the nose during the quick stop? 45 degrees?
Most likely Settling with Power or "over-pitching".