Just to add to Tinstaafl's comment, power available tends to increase with speed, and it's that aspect that causes Vy to reduce as power generally decreases.
If power were constant wtih airspeed, Vy, the speed for maximum excess power would be equal to the power-off minimum-sink-rate speed. Generally, it's much higher than that, so the power available must increase substantially with speed.
If you scale down the power by a constant fraction (say to 75% by going to about 8000 ft in a normally aspirated engine, or by 50% by feathering one engine), the slope of the power available curve reduces in proportion. So the speed for maximum excess power will shift back towards minimum sink-rate speed.
Thus Vy decreases with altitude, and Vyse is less than Vy.
[Easiest if you draw the curves]