Melt the butter in a large non-stick skillet over medium heat. Add the apples and cook until the apples feel tender when pierced with a fork, 8 to 10 minutes. Stir in the milk. Gradually stir in the oatmeal. Now, reduce heat to low and cook until all of the liquid has been absorbed by the oatmeal, 5 to 7 minutes. Remove from heat and add the vanilla.