The downwind part is easily apparent to me, because that was not only my first frightening encounter with VR but also the probable cause thereof.
I was doing a low speed recce of a potential confined area at the base of a deep gorge, in the bend of a river and it was obvious that it would have only limited approach directions, and they would be relatively steep. There were also some power-lines around (and a flying fox wire-borne cross-gorge funicular close-by). Because I wasn't planning to terminate in an OGE hover but just do a slow flyby for my crewman to take some photos, I disregarded the wind - which wasn't really significant anyway, although it was certainly about a 5 to 9 kt downwind component....and the air was quite smooth. It was only as we came down to the level of the rim of the gorge that I started paying attention to the rate of descent, because it seemed excessive. I pulled in some collective (UH-1B) and the rate of descent and blade-slap increased quite noticeably (being below the level of the surrounding high terrain). Half my mind was concentrating on the constraints of the flight path for a good photographic pass, but the other half was becoming increasingly concerned with the plummeting aspect. Because we were quite light I wasn't really concerned at this stage, more perplexed really. At about the stage where I was beginning to get ground rush I had pulled it in to max torque but hadn't done anything really about the flight-path, still fixated on the photography aspect and still inexplicably intent on maintaining the angle so as to bypass the pad at a good ground-surface recce height. We may have actually been arcing over somewhat because of that downwind approach effect.
This all happened over the space of no more than 35 to 50 seconds and you have to remember that I'd slept through the VR and retreating blade stall lectures and still had only about 50 hours on choppers. Dumber than dog-droppings really. At the point where I threw it all away and looked desperately for survival solutions I wasn't thinking engine failure or anything machine related, because everything sounded right and felt right, it was just that we were in a hell-bound and cable-cut freight elevator. To this day I think that my instinct of "overshoot" was a valid solution (i.e. I didn't consciously lower the nose with the cyclic as an answer to VR - because I hadn't realised that that was what it was). At about the same time, around 400ft AGL the river, we entered the narrow bend of the gorge and the air changed direction and speed with a bit of turbulence - and it was as if we'd jettisoned a very heavy load. I felt a real surge of lift upon the rotors. Might have been the venturi effect of the gorge narrowing or just the change in local wind velocity but it was something akin to the Hand of God as far as I was concerned. I was quite shaken by it, later discussed it with a few people smarter than I and it all then became clear.
The real clue to the significance of the approach being downwind is that I was flying in relation to the ground and so enhancing the probability that I'd be staying in my own downwash. Highly probable that it was an arcing over approach because of the overshoot trend effect of the downwind component. The very same consideration applies to what happened to the MV-22 at Marana, except that he
arranged for the downwash to be on his flight-path by fully utilising that 95 degree nacelle-tilt capability (to kill his wingman overshoot). Think about just what that extra 5 degrees does. Great for slowing down, but it also puts your downwash right in place, beneath and ahead, ready for recirculation.
So I think that
"downwind" is very relevant to your chances of encountering vortex ring. Been there, done that, still got the stained undergarment.
Those unfamiliar with the night-crash of the Puma VH-WOF on oil-rig approach to a tanker's helideck off the Coast of Western Australia might care to download the 170 kb file from
this url. It has some good lessons to be learned, first person commentary in there (as I recall). It may have been that a moving vessel's deck has the same effect as a downwind component? Too late to think that one through. Anyway, it sucked in two experienced rotary drivers, so it's worth reviewing.