In small bowl, mix brown sugar and pumpkin pie spice; set aside. In small microwave-safe dish, microwave butter on High about 30 seconds or until melted. Stir in vanilla. In large microwaveable bowl, mix all cereals and pecans. Pour butter mixture over cereal mixture, stirring until evenly distributed. Add sugar and spice mixture and stir until coated.
Microwave uncovered on High 5 minutes or until mixture begins to brown, stirring every minute. Spread on wax paper or a cookie sheet to cool. Store in airtight container.
