vast is used to describe truly large areas of space: oceans, countries, deserts, outer space tremendous is not normally used to describe the physical size of something, but a quantity or amount, i.e. "the tremendous support we've received" "a tremendous effort" "a tremendous amount of money". The meaning is usually positive huge can describe anything that is physically bigger than it should be, like a person, or a part of person. An ant that is an inch long would be a huge ant. Can sometimes be interchanged with tremendous, i.e. huge amount of money, huge effort, but without the positive connotation, i.e. a huge mistake.