Vashon, Washington (Wikipedia) Vashon is a census-designated place (CDP) in King County, Washington, United States. It covers an island alternately called Vashon Island or Vashon–Maury Island, the largest island in Puget Sound south of Admiralty Inlet. The population was 10,624 at the 2010 census and the size is 36.9 square miles (95.6 km2).