In a small, heavy bottomed saucepan over medium heat, combine 1 ½ c sugar, corn syrup, and the water; cook, stirring until sugar has dissolved, about four minutes. Raise heat; bring to a boil without stirring, washing down sides of pan with a pastry brush dipped in water. Continue cooking until thermometer reads 230 degrees, about 5 minutes.
Meanwhile, in the bowl of an electric mixer fitted with the whisk attachment, beat egg whites on medium until soft peaks form, 2 to 3 minutes. Gradually add remaining ¼ c sugar. Reduce speed to medium-low. When syrup has reached 230, remove from heat; with mixer on low, carefully pour syrup in steady stream down side of bowl.
Continue beating on medium until completely cool, thick, and shiny, about 7 minutes. Beat in vanilla. Use immediately.