vandalia is a city in fayette county, illinois, united states. it is northeast of st. louis, on the kaskaskia river. it was the capital of illinois from 1819 to 1839. (the state legislature moved the capital from kaskaskia to a more central location in vandalia.) it was also the west end of the national (cumberland) road, which is now u.s. highway 40. today it is the county seat of fayette county and the home of the vandalia state house state historic site (1836). the population was 7,042 at the 2010 census.