Preheat oven to 300. Remove all unpopped kernels from popped corn. Put popcorn in a large roasting pan; keep warm in oven. Butter the sides of a 2-quart saucepan. In it combine sugar, water, corn syrup, vinegar, and salt. Cook to 270 deg (soft-crack stage), stirring frequently (mixture should boil gently over entire surface). Remove from heat; stir in vanilla. Slowly pour mixture over hot popcorn. Stir just until mixed. Butter hands; using a buttered cup, scoop up popcorn mixture. Shape with buttered hands into 2-1/2 to 3-inch balls.