Put apples, brown sugar, cider, lemon juice, cinnamon, nutmeg, cloves and salt in a 6-quart slow cooker. Toss until the spices are dispersed.
Cover and cook on LOW for 7 to 8 hours, stirring once or twice if convenient, until the apples collapse when pressed.
Turn off the cooker briefly and blend with an immersion blender until completely smooth, keeping the blade head submerged.
Return to LOW with the lid offset slightly so steam can escape. Cook 1 to 3 hours more, stirring every 30 minutes, until a spoonful holds a soft mound.
Stir in vanilla. Taste after cooling a spoonful; add a pinch more salt or lemon only if needed.
Transfer to shallow clean containers, cool promptly and refrigerate. This recipe is not a shelf-stable canning formula.