The reason you won't get VRS in an updraught by a cliff face/mountain is that the all the air isn't going even close to straight up so you won't have the low IAS required.
The updraught is caused by the horizontal movement of air, that is the air all the way from the bottom to the top of the cliff being forced up and over it, so whilst you may have very low power (even autorotation) due to the vertical component, the horizontal component of the wind at the top of the cliff where you are hovering is going to be too high to give you VRS problems.
This is not the same as descending vertically (or nearly so) in a uniform airmass.