You got it right.
A zoom climb is not sustained like Vx and Vy are.
However if we want to be accurate, even if I agree with what you mean, NO climb speed, including best angle and max rate are sustainable on our aircrafts...
A "zoom climb" as you call it is sometimes the real answer to an emergency (wind shear at very low altitude, stall at very low altitude, imminent crash...) to avoid terrain, best angle speed won't be the one we will be looking for in those cases, the procedure will be to pull up to stick shaker (close to stall speed) to get the actual "best angle" and save your life, because the best angle speed remains in the real life a theorical speed that doesn't fit all real situations requesting the actual best angle performances from your airplane.