Should hot water be added to the already hot cooked vegetables for making a curry or even room temperature/cool water would do?
This is my personal experience. It takes sometime to reach the boiling temperature (again) when you add colder water. As a  result, vegetables get cooked for a longer period than anticipated.  This makes some vegetables becoming mushy and not tasting as good as otherwise. (Example: Egg Plant)  Therefore, I make sure to add the right amount of water in the first place.