Prep the Apples: Peel, core, and slice the apples thinly and add them to the slow cooker.
Combine: Add the brown and granulated sugars, cinnamon, nutmeg, allspice, salt, lemon zest and juice, and vanilla to a bowl and mix to blend. Add to the slow cooker and stir to coat the apple slices.
Slow Cook: Cover the slow cooker and cook on low for 8 to 10 hours or on high for about 5 hours. The apples should be very tender.
Process: Use an immersion blender to blend the apples and liquids until smooth, or transfer everything to a blender and pulse until smooth. If the mixture is too thin, put it back in the slow cooker and cook on high until thickened.