Six to eight hours before preparing, freeze the grape juice in an ice cube tray to make about 4 cubes.

To prepare frappé, combine grapes, buttermilk, 1 1/2 tablespoons of the sugar, and the vanilla in a blender. Blend until very smooth. Add grape juice cubes and blend until smooth. Taste and blend in additional sugar if necessary. Strain if desired and divide into chilled glasses.