Original Message

Vy is best rate of climb. Maintain to reach desired altitude in the least amount of time. It will also be your maximum endurance speed, to give you the most time orbiting (not to be confused with max range). Vtoss is takeoff safety speed, specifically for multi engine aircraft. If you lose an engine above Vtoss, you (in theory) can continue takeoff and fly away. Losing an engine below Vtoss would result in being forced to land. Both speeds change based on weight and density altitude.