In a 1 quart microwave safe bowl, combine chocolate chips, peanut butter and margarine. Microwave for 1 to 1 1/2 minutes or until smooth, stirring after one minute. Stir in vanilla extract. Pour chocolate mixture over cereal. Stir to coat all pieces. Pour cereal into a resealable plastic bag with powdered sugar. Seal securely and shake to coat all cereal with powdered sugar. Spread on waxed paper to cool. Makes 9 cups chex mix (about 9 servings).