Combine one 11.5-ounce can thawed frozen purple grape juice concentrate, 1/2 cup water and 1 teaspoon vanilla in a medium skillet; cook over medium-high heat until syrupy, about 10 minutes. Stir in 1 more teaspoon vanilla; let cool. Add seltzer.
For each drink, combine 2 to 4 tablespoons flavored syrup with 1 cup cold seltzer in an ice-filled glass.
Photograph by Charles Masters