Grease 9 x 13 inch pan. Place the cereal in a large bowl.

In a saucepan, combine syrup, butter and chips. Heat to boiling, stirring constantly. Remove from heat, stir in vanilla.

Pour mixture over the cereal in the bowl, toss until coated. 

Fold in marshmallows, 1 cup at a time.

Press into pan with buttered back of spoon. Let stand 1 hour. 

Cut into squares. Store at room temperature.