TC, with warmest regards ....
I spent a few years flying helicopters off the back end of ships, sometimes in the dark. One of the well known traps of approaching the ship at night, when it was moonless and cloudy, pattern altitude at 400' or a bit lower if the ceiling was coming down ... and you are flying in from about a mile and a half, was that "hole" or that "sudden sinking feeling" that came at somewhere between half a mile and an eighth of a mile, depending on ship speed and wind velocity and direction, and how well you were on profile.
As you slowed down to reduce your closure rate, you'd lose translational lift. If you weren't staying a bit ahead of the aircraft, or were fixating on the line up or on one instrument, or something distracted you ... as you slowed down you could begin to drop toward the ocean, particularly if you were a few knots fast and had reduced collective and raised the nose a bit to adjust closure/speed. More than once I hollared for power from the PNF seat (and more than once I pulled up on the collective while hollaring, better to sort it out later ... ) often in duet with the crewman. I had it hollared at me a few times as well.
That "falling into the black hole" had nothing to do with vortex ring state.
It had to do with lift loss at low altitude due to decel, reducing power, and losing the benefit of translational lift at a time when outside references didn't quickly cue you to the aircraft beginning to settle.
The mishap aircraft in this case was low altitude, crap weather, few to visual cues of settling ... I don't know, I am guessing but I suspect poor. If the crew were mostly letting Otto fly, or HAL, or whomever, and Otto was the one leading the decel (or a decel sooner/faster that should have been ... ) the reaction time to "Hey, we are falling arse first into the black hole" and correcting with power and pitch would be a bit delayed compared to if they were flying manually.
VRS?
Not necessary to fall arse first into the hole left by a lack of lift at that transition zone.
If you delay the reaction, you get a RoD which you may not arrest with that armpit of collective a few seconds later. Momentum and all that.
While I confess to you that the above includes some guesswork, that regime of flight need not get into VRS in order to end up falling down and arse first when one did not intend to.