Wash and trim the rhubarb, then chop it into roughly 1-inch pieces. Place the chopped rhubarb in a large, clean glass jar or container with a tight-fitting lid.

Pour the vodka over the rhubarb. Add the split vanilla bean (if using) and the lemon peel strips. Seal the jar tightly.

Store the jar in a cool, dark place and let it infuse for at least 10 days, or up to 4 weeks. Shake the jar gently every few days.

After the infusion period, strain the mixture through a fine-mesh sieve lined with cheesecloth into a clean bowl or pitcher. Discard the rhubarb solids, vanilla bean, and lemon peel.

In a separate saucepan, combine the granulated sugar and water. Heat over medium heat, stirring until the sugar is completely dissolved and a simple syrup forms. Let it cool completely.

Add the cooled simple syrup to the strained rhubarb-infused vodka. Stir well to combine. Taste and adjust sweetness if desired by adding more simple syrup, then let it rest for another day or two.

Pour the finished rhubarb liqueur into clean bottles. Seal and store in a cool, dark place. It will keep for several months.