In a medium bowl, whisk together vinegar, water, sugar, and salt until sugar and salt are dissolved, about 1 minute. Add apple and stir to combine. Cover and refrigerate until tart and crisp, about 1 hour or up to 2 weeks.
In a large bowl, whisk together yogurt or sour cream, mayonnaise, chives, salt, and 1 tablespoon of the reserved pickled apple brine until smooth. Add grapes, walnuts, celery, and pickled apples and stir until well combined. Garnish with celery leaves and serve immediately.
