Do The UK Military teach Vortex Ring?
As an ex-Wasp pilot I can well imagine the VRS Scout disappearing out of camera shot very quickly. It was demonstrated to us in type training & required as a part of annual competency, purely because the aircraft lost so much altitude sooooo quickly.
Even so, the first time I saw it for real (B206L1 on a Pacific Is at sea level & sufficiently below mauw) it was all over in the blink of an eye, the ground rose extremely quickly & the bang at the bottom was hard. I flew it away several days later after the engineers had had their tuppence worth.
As a result, I've included the exercise in every type rating & for every PPL/CPL, R22, B206, H300/500, AS350, UH1/204/205/212/222, AH1 Scout etc - as a part of FTH, topping check, RBS, high altitude handling etc. Don't know that I'd like to do it at some of the low altitudes mentioned here though - once bitten, twice shy.