In a large greased roasting pan, combine the popcorn, cereal, and peanuts. In a large saucepan, combine the brown sugar, corn syrup and butter: bring to a rolling boil. Boil for 3 minutes, stirring occasionally. Remove from the heat; quickly stir in vanilla and baking soda until mixture is light and foamy. Immediately pour over the popcorn mixture; toss to coat. Bake, uncovered, at 250 degrees for 1 hour, stirring every 15 minutes. Stir in candy pumpkins. Cool completely.