Prepare the Honey-Lavender Simple Syrup: Bring water, honey, and sugar to a boil over medium-high heat. Add lavender. Reduce heat and simmer until thickened, about 10 minutes. The syrup is done when it coats a spoon. Remove from heat and pour through a fine-mesh strainer to remove lavender.
Combine juice, vodka, and simple syrup in large pitcher or bowl and chill. Add champagne just before serving.
Makes about 1 cup of simple syrup | Makes 8 cups cocktail