Mostly from Ray Prouty
VY occurs when the power required curve is at its lowest point, when most excess power is available, or where the gap between the two curves is the largest. As you increase altitude, the power required curve moves up and to the right, firstly because TAS increases relative to IAS, but also (as power is drag x TAS), the power required increases as well. VY must also increase because the graph moves. Having said that, as air density decreases, IAS reduces against TAS quicker than TAS increases, so the indicated speed for VY decreases slightly (VY is based on IAS). It also increases with weight, because you need more TRT, so more collective means more induced power, more tail rotor thrust, increasing profile power. In short, when altitude increases, TAS increases but VY as a function of IAS decreases.
As TAS, VY increases, hence the confusion on some charts.