Vernon County suffered considerable damage during the American Civil War. The courthouse was burned by Union Army soldiers on May 23, 1863, along with the entire town of Nevada. The present courthouse dates to 1908. Vernon County was one of four Missouri counties that were completely depopulated by Union General Thomas Ewing's notorious General Order No. 11 (1863). Most of the inhabitants would not be allowed to return to their homes until after the Civil War ended in May 1865.
According to the 2000 U.S. Census, the county has a total area of 837.04 square miles (2,167.9 km2), of which 833.95 square miles (2,159.9 km2) (or 99.63%) is land and 3.09 square miles (8.0 km2) (or 0.37%) is water.
There were 7,966 households out of which 32.20% had children under the age of 18 living with them, 55.30% were married couples living together, 9.60% had a female householder with no husband present, and 31.80% were non-families. 28.10% of all households were made up of individuals and 13.00% had someone living alone who was 65 years of age or older. The average household size was 2.44 and the average family size was 2.97.
In the county the population was spread out with 26.60% under the age of 18, 9.20% from 18 to 24, 25.40% from 25 to 44, 22.50% from 45 to 64, and 16.30% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 93.50 males. For every 100 females age 18 and over, there were 88.00 males.
The median income for a household in the county was $30,021, and the median income for a family was $37,714. Males had a median income of $28,182 versus $19,026 for females. The per capita income for the county was $15,047. About 10.10% of families and 14.90% of the population were below the poverty line, including 20.30% of those under age 18 and 13.30% of those age 65 or over.