Combine sugar, water, and lime zest in a small saucepan over high heat. Bring mixture to a boil, stirring constantly. Boil for 2 to 3 minutes or until sugar is dissolved. Remove from heat and cool completely. Strain syrup through a wire-mesh strainer, discarding the zest.
In a blender of food processor, combine diced watermelon and simple syrup and blend until mixture is pureed. Add vodka, stirring to combine. Cover and refrigerate for 2 hours.
Pour mixture into an ice-cream maker and freeze according to the manufacturer's instructions. When sorbet is frozen, remove from ice cream freezer and place into individual paper cups and freeze until firm, about 4 hours. Drizzle additional vodka over sorbet just before serving, if desired.
Yield: about 7 cups sorbet, serving 4 to 6