What would happen to the vacuoles of a cell of a plant which has not been watered?

As such, vacuoles have no effect on them when plants are not watered. Vacuole just change their shape. There could be greater effect on the whole plant, when it is not watered. Rate of photosynthesis will get lower and wilting of plant parts such as leaves may also occur. Wilting occurs when vacuoles are not expanded  to provide turgidity to cells and plant parts due to presence of less water.