Butter rectangular pan, 13x9x2 inches. Heat corn syrup, margarine and chocolate chips to boiling in 3-quart saucepan, stirring constantly; remove from heat. Stir in vanilla. Pour over cereal in large mixing bowl; toss quickly until completely coated with chocolate. Fold in marshmallows, 1 cup at a time. Press mixture evenly in pan with buttered back of spoon. Let stand until firm, at least 1 hour. Cut into about 1 ½ -inch squares. Store loosely covered at room temperature. 48 squares.