In a 13-in. x 9-in. baking pan, combine cereal and pecans; set aside. In a microwave-safe bowl, combine the brown sugar, corn syrup and butter. Microwave, uncovered, on high for 2 to 2-1/2 minutes or until mixture comes to a boil, stirring occasionally. Stir in vanilla and baking soda.
Pour over cereal mixture; stir to coat evenly. Bake at 250° for 1 hour, stirring every 20 minutes. Turn onto waxed paper to cool. Break into bite-size pieces.