Lightly salt popcorn; keep warm. In a small saucepan, combine sugar and corn syrup; cook and stir over medium heat just until mixture comes to a full rolling boil, 3-4 minutes. Remove from heat; stir in peanut butter and vanilla. Immediately pour over warm popcorn, mixing gently until all is coated. With well-buttered hands form into balls, using about 1/2 cup mixture for each. Wrap in waxed paper or plastic wrap; tie to close, if desired.