Original Message

This is my take on the subject.

Vy: Best rate of climb, speed for minimum power, sum of all drags at a minimum (bucket speed), typically flown 60-80 KIAS depending on aircraft frontal area/size/drag factor. 

Vtoss/Vx: Best angle of climb, slight back side of the power curve, lower value than Vy, typically 40-50 KIAS depending on aircraft frontal area/size/drag factor. 

Both Vy and Vtoss are typically flown at the maximum power available.

Vy: Think POWER/DRAG (i.e. the aircraft climbs to altitude fast, pilot is exploiting the minimum drag acting on the aircraft while applying maximum power).

Vtoss: Think POWER/MAIN ROTOR THRUST (i.e. the aircraft climbs steeper, pilot is maximizing the main rotor thrust vector at maximum power).