Bring the granulated sugar and 1/2 cup water to a boil in a small saucepan, stirring until the sugar has dissolved. Halve the vanilla bean lengthwise, scrape out the seeds with a knife, then add the seeds and pod to the saucepan. Boil the syrup for 1 minute more. Let cool, then transfer the syrup to a 3-quart pitcher.
Add the Lillet, soda water and 2 tablespoons of the lemon juice to the pitcher, and stir to combine. Add the starfruit, and cover the pitcher with plastic wrap. Refrigerate the sangria to chill, at least 4 hours up to overnight.
When ready to serve, put the remaining 1/2 tablespoon lemon juice on a saucer. Put some sanding sugar on another saucer. Dip the rims of 8 glasses in the lemon juice, then in the sugar. Add the champagne to the pitcher, and stir gently to combine. Fill each glass halfway with ice, then pour sangria and starfruit into each glass.
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