Prepare the chicken by cutting the 2 large precooked breasts into bite-sized cubes and place them in a large mixing bowl.
Add the chopped onion, chopped celery, halved red seedless grapes, roasted cashews, and chopped flat-leaf parsley to the bowl with the chicken.
In a small bowl or measuring cup, whisk together the mayonnaise, white vinegar, granulated sugar, salt, and black pepper until smooth.
Pour the dressing over the chicken mixture and toss gently until everything is evenly coated.
Taste and adjust seasoning with more salt or pepper if desired, then serve on bread, with crackers, or plain.