Discard unpopped popcorn kernels. Spray a 17x12x2-inch roasting pan with nonstick cooking spray. Place the popped corn and pecans in the pan. Keep popcorn warm in a 300 degree F oven while making coating.
In a small saucepan melt the margarine or butter. Remove saucepan from heat. Stir in the corn syrup, pudding mix, and vanilla. Pour syrup mixture over popcorn. With a large spoon, gently toss the popcorn with the syrup mixture to coat.
Bake popcorn, uncovered, in a 300 degree F oven for 16 minutes, stirring halfway through baking. Remove the pan from the oven. Turn mixture onto a large piece of foil. Cool popcorn completely. When cool, break into large pieces. Store leftover popcorn, tightly covered, in a cool, dry place for up to 1 week. Makes about 9 (1-cup) servings.