Insert craft sticks into stem of apples. Mix all the ingredients, except the vanilla and nuts, in a large saucepan: cook, stirring constantly, to 240 degrees F on a candy thermometer. Remove from heat and let cook until mixture thickens. Add vanilla. Quickly dip apples in the mixture and twist until apples are evenly coated. Place on waxed paper to drain and firm up. If desired, roll candy apples in chopped nuts.