Remove all unpopped kernels from popped corn. Put popcorn into a 17x12x2 baking pan. In a 1 1/2-quart saucepan combine sugar, butter or margarine, corn syrup, and salt. Cook and stir over medium heat till butter melts and mixture comes to a boil. Cook, without stirring, for 5 minutes more. Remove from heat. Stir in baking soda and vanilla. Pour over popcorn, stirring gently to coat popcorn. Bake at 300 for 15 minutes. Stir, then bake for 5 to 10 minutes more. Remove corn to a large bowl and allow to cool.