Combine water and sugar in a small saucepan over low heat. Heat, stirring, until the sugar has melted. Remove from heat; refrigerate.
Put cherries in a bowl. Squeeze lime juice over top and stir to make sure the cherries are coated in juice. Stir in syrup; refrigerate at least an hour.
Stir cherry mixture and vodka together. Fill glasses with ice. Put ½ cup of cherry mixture into each glass. Top with sparkling water; serve immediately.