Spray an 18x12x2-inch baking pan with nonstick spray. Remove unpopped kernels from popped corn. Place popcorn in pan; keep warm in a 300 degree F oven.
Butter sides of heavy 1-1/2-quart saucepan. Add brown sugar, butter, and corn syrup. Clip candy thermometer to side of pan. Cook and stir over medium heat to 255 degree F, hard-ball stage (about 4 minutes). Remove saucepan from heat. Stir in baking soda and vanilla; pour over popcorn. Stir gently to coat.
Bake in a 300 degree F oven for 15 minutes. Stir and bake 5 minutes more. Remove from oven; spread onto a large piece of foil and cool completely. Break into clusters. Makes 9 cups.