Preheat the oven to 350°. In a bowl, toss the grapes and vinegar. Let stand 15 minutes.
Meanwhile, put the walnuts in a pie plate and bake for about 6 minutes, until lightly browned. Let cool, then coarsely chop.
Drain the vinegar from the grapes into a small saucepan. Add the sugar and cook over moderate heat until syrupy, 2 minutes. Pour the syrup over the grapes, toss and let cool. Fold in the walnuts just before serving.