You can use it to indicate the amount or size of something that is very large. For example, "The warehouse held a vast quantity of furniture.". That's a vast quantity of ice.

You can use it when you are referring to a very large quantity of something. For example: "The students had to read a vast amount of articles for the assignment." That's a vast amount of work.

If an area is very large, or there is a very high number of something, you can say it is "vast." For example, the ocean, the Sahara Desert, and outer space are all vast.