Vandalia
- County:
- Fayette County
- County Seat:
- Yes
- Area (mi²):
- 8.147
- State:
- Illinois
Vandalia is a city located in Fayette County, Illinois. Vandalia has a 2026 population of 7,358. It is also the county seat of Fayette County. Vandalia is currently declining at a rate of -0.24% annually and its population has decreased by -1.31% since the most recent census, which recorded a population of 7,456 in 2020.