Aye cheers for that nick, just checked the Principles of Flight book there..
Vortex State Ring can develop when the helicopter has:
1 A low or zero airspeed (below translational lift speed)
2 Some power in use (collective input) 'As you mentioned'
3 A rate of descent that is in the region of 400 to 800ft/min, depending on aircraft type!
Duno were i got 30 Knots out of!