Coalface,
The formula is one that assumes the rotor produces a uniform stream of moving air, and the velocity is a constant, based on the weight of the machine (thrust produced).
In reality, there is a distribution of little streams within the wake that have various velocities, and these can be considerable greater. I would not be surprised to find strands of 60 knots but would be surprised if they were very large. On the other hand, when you are on the ground and the stream has bent 90 degrees along the ground, especially if a wind has helped concentrate the flow, then there might be plenty of concentration.
In measurements of the S-92, we found knots mostly in the 40's, which is close to the theoretical.
Remember that if the downwash is faster, this would make VRS even less likely, which is what sparked this whole thread.
Happy landing! The VRS is totally unrelated to rotor stall, which occurs at maximum lift for the blade, usually near maximum speed. I can popst some of that stuff if you'd like.
Nick