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.
Put lime juice into a medium nonreactive bowl. Halve and pit cherries; add to lime juice. Stir in syrup. Refrigerate at least 1 hour (up to overnight).
Stir cherry mixture and vodka in a large serving bowl. Fill 12 glasses with ice. Spoon 1/2 cup cherry-vodka mixture into each glass. Top off with sparkling water. Serve immediately.