To prepare the peaches, simply remove pit and puree in blender or food processor.
Split vanilla beans in half and scrape beans from pod. Place all ingredients into a large stockpot, including scraped vanilla pod. If the capacity of your stockpot is not sufficient, you may have to wait for adequate reduction before adding all the desired sweetener or lemon juice.
Reduce in a partially covered slow cooker crock on low heat for 12-18 hours. Stir somewhat infrequently, allowing browning and carmelization around the edges. When peach butter is reduced by approximately 1/3, remove spent vanilla pods and discard.
Scrape down side of crock. To ensure smoothness, re-puree with an immersion blender.
Ladle into sterilized half pint jars, leaving 1/2" head room. Wipe rims clean and place prepared lids and rings onto jars, finger tight.
Process in a water bath canner for 10 minutes at a full boil. After processing, remove jars from canner and allow to cool . undisturbed, for 24 hours. Check for seal and store in a cool and dark place for up to a year.