Rotorbee - the reason that the diagram has a dotted line marked autorotative state and windmill brake state is that in a vertical descent (in auto) the rotor is slowing down the air passing through it and extracting the energy to keep the rotor turning. As such there is a limit to the RoD in auto because of the size of the rotor disc - although some of the inboard section is causing rotor drag, the outer areas are producing rotor thrust (with the tips creating drag again.
The point is that essentially none of the disc is stalled (apart from the absolute root) and it is all providing a useful function in slowing the RoD.
Now in VRS, when the root end of the blades are stalled due to high AoA (collective raised) and the outboard section is enveloped in a very high drag vortex, only a small part of the disc (a ring between the root and the tips) is producing lift and/or slowing the airflow down.
The more the lever is raised, the worse it gets so the size of your rotor disc (or that part that can slow your RoD) is greatly reduced, leading to much higher rates of descent than can be experienced in auto.
This is why in FDVRS there are the stories of 3 - 5000'/min RoD and why it is so deadly - and why the VT just won't cut it in FDVRS.