Toast the walnuts: Add chopped walnuts to a dry skillet over medium heat.
Stir for 3–4 minutes until fragrant and lightly browned. Transfer to a plate to cool.
Prep the grapes: Rinse and pat dry. Slice each grape in half.
If your grapes are very large, quarter them.
Make the dressing: In a medium bowl, whisk together Greek yogurt, honey or maple syrup, lemon juice, Dijon, vanilla (if using), salt, and pepper until smooth.
Combine the salad: In a large bowl, add grapes, celery, and most of the walnuts. Pour the dressing over and gently fold until everything is coated.
Taste and adjust: Add a pinch more salt, a squeeze of lemon, or a drizzle of honey to balance the flavors.
Finish with herbs: Fold in chopped mint or parsley. Sprinkle the remaining walnuts on top for crunch.
Chill (optional): Refrigerate for 20–30 minutes to let the flavors meld.
Serve cool.