Have ready 4 quarts popped popcorn. In saucepan combine 1/2 cup sugar and 1/2 cup light corn syrup. Heat this to a good rolling boil and remove from heat. Stir into this 1/2 cup peanut butter, 1/2 teaspoon vanilla and 1/8 teaspoon salt. Pour over corn, stirring until well coated. Shape into balls.