albatross,
I think the explanation is that it entered Power Settling and then degraded to VRS as the power-induced descent increased. The terrible thing about rotors is that as you go from a stable OGE hover downward, the power needed to hold a steady slight rate of descent is HIGHER than the power needed for steady hover. In effect, a piece of VRS is introduced to the rotor and the pilot must raise the collective to add some torque to hold the slight descent. If you don't raise the collective, the slight power deficit of the descent makes the descent rate increase, and down you go.
AND our training has said that because VRS is the dreaded beast, never raise the collective in a vertical descent. Well, that is bunk. If you have the engine power, raise the collective and stop that moderate descent. Up to descents of maybe 700 to 2000 feet per minute (depending on the disk loading and downwash velocity of your helo) you have NO danger of VRS at all.
In short, up to horrendous vertical rates of descent, the issue is PERFORMANCE and ENGINE POWER, not VRS.