Vandalia is a city in Fayette County, Illinois, United States. At the 2010 Census, the population was 7,042. Vandalia is 69 miles (111 km) northeast of St. Louis, on the Kaskaskia River. It served as the state capital of Illinois from 1819 until 1839, when the seat of state government moved to the current capital of Springfield. Vandalia was for years the western terminus of the National Road.
Today, Vandalia is the county seat of Fayette County and the home of the Vandalia State House State Historic Site (1836).