Coat a 9 x 13 inch pan with spray oil or margarine.
In a large saucepan, melt the butter or margarine over low heat. Add the vanilla. Melt the marshmallows into the margarine, stirring. Add the cereal when the marshmallows have melted; stir until cereal is coated. Quickly pour into the prepared pan. Use a sheet of waxed paper to press the mixture down flat and evenly into the pan.