Grease three 15x10x1-in. baking pans; set aside. In a large heat-proof bowl, combine the cereals; set aside.
In a heavy saucepan over medium heat, bring the butter, brown sugar and corn syrup to a boil, stirring constantly. Reduce heat. Simmer, uncovered, for 5 minutes, stirring occasionally. Remove from the heat; stir in vanilla. Pour over cereal and stir until well coated.
Transfer to prepared pans. Bake, uncovered, at 200° for 1 hour, stirring every 15 minutes. Cool completely. Break apart and store in airtight containers.