Vandalia is a city in Montgomery County, Ohio, United States, and a suburb of Dayton. Its population was 15,209 during the 2020 census…Vandalia is a city in Montgomery County, Ohio, United States, and a suburb of Dayton. Its population was 15,209 during the 2020 census. In addition to being the city closest to Dayton International Airport, Vandalia lies at the crossroads of I-75 and I-70.