Preheat oven to 300 degrees. After popping, discard unpopped popcorn kernels. Spray a 17 x 12 x 2-inch roasting pan with nonstick cooking spray. Place the popped corn and pecans in the pan. In a separate pan, heat butter and corn syrup, then stir in pudding mix and vanilla. Pour mixture over popcorn. Bake in oven at 300 degrees for 16 minutes, stirring halfway through baking. Remove pan from oven and turn mixture onto a large piece of foil. Cool popcorn completely. When cool, break into large pieces and serve.