In small bow, mix brown sugar and pumpkin pie spice, put off to side. In small microwave-safe dish/bowl, microwave butter on High about 30 seconds or until melted. Stir in vanilla. In large microwavable bowl, mix all cereals and pecans. Pour 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.