Reserve 1 cup of the marshmallows. In 3-quart saucepan, heat remaining 4 1/2 cups marshmallows, the chocolate chips, butter, sugar and water over low heat, stirring occasionally, until completely melted. Remove from heat; stir in vanilla. Pour marshmallow mixture over cereal in bowl; stir until evenly coated. Stir in remaining 1 cup marshmallows.