For the Illinois village formerly known as Vienna, see Astoria, Illinois Vienna is a city in Johnson County, Illinois, United States. The population was 1,234 at the 2000 census. It is the county seat of Johnson County, and the site of a well-known state penitentiary. Heron Pond - Little Black Slough Nature Preserve, a National Natural Landmark, is nearby. The town's name is pronounced differently from the English name for the Austrian capital of the same name.