Place popcorn in a large heat-proof bowl; remove any unpopped kernels and set aside. In a large saucepan, combine remaining ingredients except vanilla. Cook over medium-high heat, stirring occasionally, until mixture reaches the soft-crack stage, or 270 to 289 degrees on a candy thermometer. Remove from heat; add vanilla and stir well. Pour over popcorn, mixing until coated. Spread on wax paper to dry.