Crab, I agree.
But:
With adding power I pump more energy into the vortex, how much does the vi change? Can you achieve a vi of 2000ft/min to keep the rotor in the shaded area? Or is the added power used, just to overcome the added drag of the stalled area?
One can assume, that the vortex will always be there, even at the highest RoD, because otherwise the rotor would completely stall and that would be the end of it. That does not happen as we know. But how big is it and where is it.
A bigger stalled area or a bigger vortex would be hard to tell apart, with all the turbulence produced from the fuselage and stalled area.
I am not absolutely convinced, that at high RoD, where Vd/vi is higher than 1.5 (there is a limit to vi) the vortex is still the problem, because it should by now be blow above the disk, as wind tunnel test show, where it's influence would be greatly diminished, because the following blade would not hit it smack on to pump energy into it, but I would still have all the ingredients of the controls not responding and so on, due to the stalled area. Which would not go away, but would get bigger.
In that case the VT would not work as advertised, it would just add a bit TR-trust to help speed up the ship to get the stalled area flying again, because that is still the whole point, to stop the descend. But since we are talking about a disk, it does not care about left, back or front. I suspect that moving the ship sideways (crab) is just faster with the help of the tail rotor than nosing over.
Instead in the darker area, I have the perfect situation to pump the most energy into the vortex with each blade, because the ship sinks too fast for the vortex to sink away and not fast enough to get the vortex blown above. That's why in that area you get the worst turbulence and control problems (the biggest and meanest vortex). If you move the vortex off the tip inside, it will be blown away pretty fast. How much control I would have left there, is not something I can find out. Reports of flight test show, that it is extremely difficult to stay inside and a very disturbing experience.
As it happens, in an early NACA paper, the dark grey area was a half circle starting at zero forward air speed. Flight and wind tunnel tests have shown, that this isn't the case and now it is between 70° and 40°. That's a problem I still have to get my head around.
Anyway, I still think the VT solves a artificially blown up problem. It is a trick of last resort.