Thank you. What I was missing is that Vy is a function of TAS instead of IAS.
Now, according to the sample charts for the sample helicopter, Vy (TAS) increases with altitude, for example from 59KTAS at MSL to 82KTAS at 9,000ft AMSL, looking at the red lines. By rule of thumb, TAS for the same IAS should increase only by roughly 18% between MSL and 9,000ft - 59KIAS at 9,000ft should result in roughly 69KTAS at that altitude.
In other words, in the sample machine, your Vy would increase from 59KIAS at MSL, to roughly 70KIAS at 9000ft to achieve the 82KTAS.
In the AS350 models, Vy (IAS) decreases instead, from 65KIAS at MSL to 56KIAS at 9,000ft.
56KIAS at 9,000ft should give around 66KTAS, so this would tell me that Vy (TAS) barely changes in the AS350 at higher altitude, and the increase in true airspeed for a given IAS, is basically what is responsible for the decrease in Vy (IAS). If I extrapolate further, Vy per the manual is 45kts at 20,000ft, that's somewhere around 62 or 63KTAS.
In other models, perhaps the increase in Vy (TAS) and the increase in TAS are more or less the same, and they get away with specifying one Vy (IAS) for all altitudes?