Vapor lock is when the amount of air or exhaust is too great in the cylinder. When it gets compressed it is too much for the piston to compress all the way enough to start it's down stroke cycle. Therefore the piston reaches a point where it stops traveling up the cylinder and it stalls the engine.

Hydro lock is the same situation but with a liquid mixture. Such as too much fuel or oil left in the cylinder. Hydro lock is MUCH more common than vapor lock in R/C two strokes.

Both situations can cause the piston to crack, the connecting rod to bend or the head gaskets to blow out. Or all of the above at once.

To cure it you have to loosen the glow plug and let the extra pressure escape. Then readjust the fuel/air mixture.