Make the cranberry filling: In a large pot over medium-high heat, add the cranberries, sugar, lemon juice and 2 cups water. Bring to a boil, then cook, stirring occasionally, for about 15 minutes, until the mixture has thickened enough to leave a trail when you carefully run a finger over the sauce-coated spatula. Mash a few cranberries against the sides of the pan using a wooden spoon or spatula. Off the heat, add the vanilla. For a smoother texture, blend the cranberry filling using an immersion blender, if desired. Set aside to thicken while it cools to room temperature.
