Vandalia is a city in and the county seat of Fayette County, Illinois, United States. At the 2020 Census, the population was 7,458. Vandalia is on the Kaskaskia River, and 60 miles northeast of the St. Louis area. The city served as the state capital of Illinois from 1819 until 1839, when the seat of state government moved closer to the center of the state in Springfield. In the early 19th century, Vandalia became the western terminus of the National Road from the East Coast.