Combine first 6 ingredients in processor. Blend until smooth.
Transfer to 11x7x2- inch glass baking dish. Freeze until starting to set around edges, stirring occasionally, about 1 hour. Continue to freeze, without stirring, until firm, about 2 more hours. DO AHEAD: Can be made 2 days ahead. Cover and keep frozen.
Remove granita from freezer; let stand until soft enough to scrape with fork, 20 to 30 minutes. Using fork, scrape granita, forming flakes. Divide among 6 glasses.
Pour 1 tablespoon vodka over each.
Sprinkle each with chopped celery leaves and garnish with 1 whole celery leaf.