Elderberry Syrup with Vanilla
Bring the apple juice, 1 liter (approximately 1 quart) of water, the slit vanilla bean, and the sugar to a boil. Reduce the heat and simmer gently for 10 minutes over low heat. Let the mixture cool to room temperature. Rinse the lemons in hot water, pat dry, and thinly slice. Don't rinse the elderflower, just brush any debris away if necessary. Add the sliced lemons and the elderflowers to a large jar or bowl. Cover with the syrup, and let infuse for 2 days. All the prepared ingredients should be completely covered.
Strain through cheesecloth into a clean pan. Bring to a boil briefly, stirring constantly. Transfer into a clean, prepared bottle. Add a few elderflowers if you'd like, and seal tightly.