Combine sugar, water, salt, and vinegar in a saucepan.
Cook and stir over medium heat just until clear.
Then cook without stirring until syrup spins a long thread when dropped from tip of spoon (or to a temperature of 240Â°F.).
Beat egg whites until soft peaks form.
Add syrup in a very thin stream, beating constantly.
After all syrup is added, continue beating until frosting will stand in stiff peaks.
Mix in vanilla.