I'm pretty sure it is impossible to get into vortex ring in any kind of auto (except a botched power recovery) as you need an induced flow through the disk to get into vortex ring, therefore in auto, no induced flow and not possible. A low speed auto only gives you two of the requirements for VRS, ie low speed and a rate of descent. I imagine if you are pulling power to overshoot from an auto and still descending with low speed on, could be exciting, but from experience, some helos are difficult to get into vortex ring even if you try.
Back in the days of gazelles and 705sqn we used to do zero speed autos by plummetting down at about 40kts indicated, with the engine at idle, wait until the requisite moment and cushion like a b#stard running along the ground at about 30kts. VERY sporting and I'm sure the instructors didn't get paid enough.
Never hit VRS in that case anyway.
Oops,
just read Mr Lappos' reply above, all covered...