Peel the apples and slice ¼ inch thick. (You should have about 9 cups.) Place the apples in a 4-quart or larger slow cooker. Add sugar and cinnamon to taste, and toss to coat well. Cover and cook until the apples are very tender and almost translucent, but not pureed, 2 to 2½ hours on high or 4 to 4½ hours on low. Stir in vanilla. Transfer to a bowl and let cool slightly. Cover and refrigerate until chilled.