Vandalia is a city in northeastern Audrain and extending into southeastern Ralls Counties in the U.S. state of Missouri. The population was 3,553 at the 2020 census, which includes about 1,000 prisoners incarcerated at the prison located within the city limits.

Audrain County is a county located in the central part of the U.S. state of Missouri. As of the 2020 census, the population was 24,962. Its county seat is Mexico. The county was organized December 13, 1836, and named for Colonel James Hunter Audrain of the War of 1812 and who later was elected to the state legislature.