Place popcorn in a baking pan and keep warm in a 200° oven. Meanwhile, in a large saucepan, stir together the sugar, corn syrup, water, butter and salt. Cook over medium heat, stirring constantly, until mixture comes to a boil.
Continue cooking without stirring until the temperature reaches 270° on a candy thermometer. Remove from the heat. Add vanilla; stir just enough to mix. Slowly pour over the popcorn. Cool just enough to handle. Shape with buttered hands.