In a large greased roasting pan, combine popcorn, cereals and peanuts: set aside. In a large saucepan, combine brown sugar, butter and corn syrup: bring to a rolling boil over medium-low heat, stirring constantly, Cook, without stirring, until mixture turns a medium amber color, about 8 minutes.
Remove from the heat; quickly stir in vanilla and baking soda until mixture is light and foamy. Immediately pour over cereal mixture; stir until evenly coated.
Bake at 250 for about 45 minutes, stirring every 15 minutes. Spread mixture onto waxed paper-lined baking sheets. Cool completely. Store in airtight containers.