In large saucepan over low heat, melt marg. Add marshmallows, stir until melted and well blended, add choc. chips and stir till melted. Remove from heat. Stir in vanilla, add cereal stirring until well coated. Using buttered spatula, press into buttered 13x9" pan. Cool and cut into squares. I sometimes add a few more marshmellows or less cereal for real oey-goey bars, my kids love them that way.