Vx is maximum
excess thrust over thrust required for LEVEL flight
Vy is maximum
excess power over power required for LEVEL flight
What's the difference between excess power & excess thrust?
Power = Thrust * velocity
so whenever you know the thrust of an airplane, you can figure out how much power it's developing by multiplying that number by how fast the airplane is going. Thrust is the more fundamental concept, because without thrust, you have no power.
One way I think about it is that power incorporates not only the concept of thrust, but also the velocity that the thrust produces. This is useful in certain contexts, because some aircraft performance parameters depend on that velocity.
For instance, your angle of climb depends on how much excess thrust you have. The velocity of the aircraft along that flight path is irrelevant. If my climb angle is 45 degrees, it remains 45 degrees no matter how fast I'm flying.
However, rate of climb depends on power. If my climb angle is 45 degrees, the faster I fly along that flight path, the greater my rate of climb is.