Pour the pitted cherries into the sauce pan and add the lemon juice. Bring to a boil, then reduce heat and simmer about 5 minutes. The cherries should be very soft and mashable.
Mash the cherries with the potato masher to break the skins, then blend with the immersion blender until the mixture is smooth. (Or blend the cherries in a blender until very smooth.)
Return the pureed cherries to the sauce pan and add the sugar and cinnamon. Bring to a boil, then reduce the heat to low and simmer uncovered until the mixture is thick, about 25 minutes, stirring frequently.
Remove from heat and stir in the vanilla extract.
Cool completely and divide between the two freezer jars. Store jars in the refrigerator or freezer. Will keep for up to 3 weeks in the refrigerator and 3 to 4 months in the freezer.