Preheat oven to 250F. Coat the bottom and sides of a large roasting pan with nonstick vegetable spray. Place popped corn in roasting pan. In a heavy pan, slowly melt butter; stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Boil without stirring for an additional 5 minutes. Remove from heat; stir in baking soda and vanilla. Gradually pour over popped corn, mixing well. Bake for 1 hour, stirring every 15 minutes. Remove from oven, cool completely. Break apart into large chunks or bite-sized pieces.
For a "State Fair Kettle Corn" Twist, increase salt to 1.5 teaspoons and use dark corn syrup instead of light.