The only way I have been able to repeatedly demonstrate the onset of vortex ring state (and I don't want to get to the fully developed version, thank you very much), is to decelerate downwind (at a sufficient height above the ground to permit a timely recovery)
Find out where the wind is at altitude (watch the clouds drifting by, or know it from some other source), but downwind is the only way I can get this to work.
Start at 60 KIAS in level flight - reduce the power to something slightly below the power for level flight at that airspeed and maintain altitude.
As the airspeed passes through about 20 KIAS, there will be increased airframe buffeting, unlike any you get anywhere else. The helicopter may have uncommanded pitch, roll and yaw (i.e. you're not moving the stick and it's dancing around on you). The rate of descent will be 500-800 fpm, maybe less.
Increasing the collective might increase the rate of descent, but this is not very repeatable.
WHen you've had enough, smoothly, but positively lower the nose to 20-30 degrees below the horizon and fly out of it.
Doing it into wind won't work.