Place the berries, honey, lemon juice, and vanilla in a blender on high and puree until smooth. Pour into a pot and bring to a boil over medium heat. Reduce heat to a low simmer and cook for 45 minutes to 1 hour, stirring occasionally, until thickened. The mixture should mound slightly when ready. Preheat the oven to 200 degrees Fahrenheit and place a silicone mat on a baking sheet. Stir in the elderberry syrup after the mixture is removed from the heat. Pour the mixture onto the silicone mat, spreading evenly to 1/8-inch thickness. The mixture may not cover the entire pan and you want to leave at least 1-inch space along all sides of the pan to prevent the mixture from seeping underneath the silicone mat. Bake for 3 hours, or until the fruit leather is dried out. The fruit leather is ready when it is slightly tacky to the touch but does not leave residue on your finger. Cool the fruit leather at room temperature for 2 hours, or up to 24 hours. Cut into individual portions, rolling each portion onto parchment paper for easy storage.