As I understand it
Now,
I dont feel fit to hold a candle to some of the contributers to this forum when it comes to technical matters but as I understand it:
Vortex ring occurs when the air gets confused as to whether it is passing down through the disk, as in normal flight or up through it, as in auto-rotation.
Descending, power off, whether at 100 kts or zero kts, does not risk vortex ring: the air goes up. Converting a zero speed auto into a normal auto does not risk vortex ring, just a very high rate of descent until normal forward speed is gained.
Arresting a vertical auto by pulling up the lever is, however a problem. The air does not know whether the engines are working or not. If there is sufficient inertia in the blades then air will be forced back down through the disk and "settling with power" will ensue even though the power is coming from the stored kenetic energy in the blades.
Expecting incoming.
OA