Measure cereal into large bowl; set aside. Microwave chocolate chips, peanut butter and margarine in 1-quart microwavable bowl, uncovered on High 1 minute; stir. Microwave 30 seconds longer or until mixture can be stirred smooth. Stir in vanilla. Pour chocolate mixture over cereal in bowl, stirring until evenly coated. Pour into large plastic food- storage bag; add powdered sugar. Seal bag; shake until well coated. Spread on waxed paper to cool. Store in airtight container in refrigerator. 9 cups snack.