Vortex ring "state" is a state that occurs when the rate of decent, and upflow from descending meet each other at a ratio. The "envelope" is very dynamic because it is a factor of rotor thrust output along with rate of descent. Typically a rate of descent that is
Between 25% and 42% of your induced flow velocity is sufficient to see indications of the incipient phase, if that ratio is allow to increase to around 75%, a fully developed vortex ring state will be encountered. The "envelope" will be larger with heavy machines that are capable of producing large amounts of rotor thrust. Butt those envelopes start a much higher rate of descent too. Small single engine piston aircraft will have narrower envelopes but the region begins at a much slower rate of descent. Therefore those operators are more likely to see VRS on a day to day basis. Since the ROD falls much closer to what is considered a "normal" rate.