Wash, peel, and slice the apples into approximately 1/4" pieces. Arrange the sliced apples evenly across the bottom of a 6 quart slow cooker. Add the brown sugar, cinnamon, vanilla, cornstarch, and cubed butter on top of the apples and stir until evenly coated. Cover and cook on HIGH for 2-3 hours, stirring occasionally, until the apples are tender and caramelized. Once tender, turn the slow cooker to WARM and serve the slow cooker "fried" apples as a side dish or dessert.