Vandalia is a city in Fayette County, Illinois, United States, 69 miles (111 km) northeast of St. Louis, on the Kaskaskia River. From 1819 to 1839 it served as the state capital of Illinois. Vandalia was for years the western terminus of the National Road. Today it is the county seat of Fayette County and the home of the Vandalia State House State Historic Site (1836). The population was 6,975 at the 2000 census.