Pour cereal into a large bowl and set aside. In a 1-quart microwave-safe bowl combine chips, peanut butter, and margarine. Microwave on high 1 to 1 1/2 minutes or until smooth, stirring after 1 minute. Stir in vanilla. Pour chocolate mixture over cereal, stirring until all pieces are evenly coated. Pour cereal mixture into a large jumbo Ziploc bag with powdered sugar in it. Seal securely and shake until all pieces are coated. Spread on wax paper to cool. Best served chilled. Great Party snack!