The quantity of telephone numbers depends on the population of the state. That is the reason why there might be only one area code in a whole state. Vermont is one of such example, when there is simply no need to issue additional area codes.
Area codes on the map of Vermont
As you can see on Vermont area code map, there is a single phone number there. 802 is the area code which covers whole territory of the state. It means that every phone number from VT starts with these digits.
Area code map for Vermont is painted in one color. If there were two or more area codes used in different parts of VT, it would be divided into several colored zones.
Area code of Vermont, VT
Area codes in other states of USA
For more information about area codes in other states or in exact cities, you can follow these links:
All area codes of USA are listed here.