1Remove rind from watermelon. Cut melon into 1-inch cubes to make 6 cups.
2In bowl or food processor, puree watermelon, sugar, lime peel, lime juice and vodka if desired for 2 minutes or until smooth. Pour mixture into large bowl. Cover and refrigerate 1 hour. Place ice-cream make bowl in freezer.
3Freeze watermelon mixture in ice-cream make following manufacture's directions for sorbet. Spoon sorbet into medium bowl. Cove and freeze 2 hours or until firm.
4To serve, scoop sorbet into 6 wine glasses. Pour 1/4 cup Asti Spumante over each and serve immediately.