Vandalia, IL has a population of 6,758 and is the 5,307th largest city in the United States. The population density is 829 per sq mi which is 273% higher than the Illinois average and 815% higher than the national average. The median age in Vandalia is 37 which is approximately 2% lower than the Illinois average of 37. In Vandalia, 39% of the population over 15 years of age are married, 95% speak English and 3% speak Spanish. 82% of Vandalia residents were born in Illinois, 17% were born out of state, 0% were born outside of the United States and 1% were foreign born.