1Combine apples and apple cider. Bring to a boil over medium-high heat. Reduce heat and boil gently, stirring occasionally, until apples are soft, about 30 minutes.
2Puree using an immersion blender, food mill, or food processor just until a uniform texture is achieved. Do not liquefy. Measure 12 cups of apple puree.
In a large slow cooker, combine apple puree and sugar. Stir until sugar dissolves. Prop lid open with using the handle of a wooden spoon and allow to reduce gently for about 8 hours.
3Stir in rum and ground vanilla and cook for 1 hour. Finished apple butter will be thick and hold its shape on a spoon.
4Can (process for 10 minutes) or refrigerate for up to 1 month. If it lasts that long.