Place the apples in a crock pot with the lemon juice. In a medium bowl, combine sugars, cinnamon, nutmeg, cloves, and salt. Pour the mixture over the apples and mix well.
Cook in slow cooker on low for about 10 hours, stirring occasionally, until the mixture has thickened and is dark brown.
Uncover, stir in vanilla, and continue cooking uncovered on low for about 2 hours.
Use an immersion blender to puree the apple butter until smooth.
Spoon the mixture into sterile jars, and process in a water bath canner for 20 minutes for half pints.