In a large microwave safe bowl, heat the marshmallows and butter in the microwave, 30 seconds at a time until melted and smooth. (more marshmallows make a stickier bar, so use 5 1/2 or 6 cups accordingly.(
Remove from the microwave and stir in the vanilla.
Let cool slightly to prevent the chocolate from melting too much and then stir in the cereal, Oreo's and the Peanut butter cups.