Combine 2 c. sugar, 1 c. water, ⅛ tsp. salt and 1 tsp. white vinegar in a heavy saucepan. Cook over medium heat, stirring constantly until clear. Without stirring, cook until mixture forms a thin thread when dropped from spoon (242° on candy thermometer). Beat 3 egg whites until stiff. Add hot syrup, beating constantly. Continue beating until frosting holds shape. Add ½ tsp. vanilla, and blend well.