My two bob's worth:
Conditions beyond which you MAY get into, at least, incipient VRS:
low ias <20 kts
highish ROD>500 ft/min
low power setting but then rapidly increasing power.
What's happening? At tips vortex ingestion reducing angle of attack=less lift - at root increasing collective pitch causes angle of attack to go beyond stalling angle=less lift therefore portion of blade producing lift is shrinking towards the middle=overall much less lift.
Situation to avoid:
Steep approach gone wrong - try to correct by dumping lever - hauling back on the stick - then "oh sh1t" now haul in lever to stop increasing ROD. Now low/slow and no room to carry out corrective action ie lower nose/lever.
Lesson: don't try and salvage a stuffed up approach - admit defeat early, swallow pride and go around.
SWP: engines have been "topped" - increasing lever beyond this point means more drag and no more engine power to sustain RRPM.
Solution:
check RFM and know where your helicopter runs out of puff IGE/OGE with respect to DA.
GAGS
E86