GREASE 13 x 9-inch baking pan. Pour cereal into large bowl.
HEAT 5 cups marshmallows, morsels, butter and corn syrup in medium, heavy-duty saucepan over low heat, stirring constantly until smooth. Remove from heat. Stir in vanilla extract.
POUR marshmallow mixture over cereal; stir until well coated. Stir in remaining marshmallows. Press mixture into prepared pan. Refrigerate for 1 hour or until firm. Cut into bars.
Microwave 5 cups marshmallows, morsels, butter and corn syrup in large, microwave-safe bowl on HIGH (100%) power for 2 to 3 1/2 minutes, stirring every minute, until smooth. Stir in vanilla extract. Proceed as above.