Preheat oven to 225 degrees. Spray 15 x 10 baking sheet with non-stick spray. Mix popcorn (make sure to remove unpopped kernels) and pecans in large bowl.
Combine brown sugar, butter and corn syrup in medium saucepan. Over low heat, stir mixture until sugar dissolves. Increase heat to high and boil 5 minutes. Remove from heat; stir in vanilla extract and baking soda.
Pour over popcorn and pecans, immediately stirring gently to coat. Pour mixture onto prepared baking sheet, spreading evenly.
Bake for 1 hour in preheated oven. Cool completely. Break into pieces and store in airtight container.