1. Bring sugar and 1 1/4 cups water to a boil in a small saucepan, stirring until sugar has dissolved. Remove from heat; let cool completely. (Makes about 2 cups syrup.) Syrup can be refrigerated in an airtight container up to 1 month.
2. Put lime juice into a medium nonreactive bowl. Halve 2/3 of the cherries; add all cherries to lime juice. Stir in syrup. Refrigerate at least 1 hour (up to overnight).
3. Stir cherry mixture and vodka in a large serving bowl.
4. Fill 12 glasses with ice. Spoon 1/2 cup cherry-vodka mixture into each glass.
5. Top with sparkling water; serve immediately.

Serves 12.

*I used plain vodka.