Coat a 20x30cm pan with cooking spray 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 Rice Bubbles when the marshmallows have melted; stir until Rice Bubbles are coated. Quickly pour into the prepared pan. Use a sheet of waxed paper to press the mixture down flat and evenly into the pan.