Pat chicken breasts dry with paper towel. Sprinkle both sides with salt and pepper. Place in vacuum seal bag, along with sliced lemon and tarragon sprigs. Vacuum seal bag. Place in water bath set to 150° F; cook for 2 to 4 hours. Remove from water bath; cool in ice bath for 15 minutes.
Add chicken, celery, mayonnaise, mustard, salt, and pepper to bowl of food processor fitted with steel blade. Pulse process 5 or 6 times to combine. Add grapes and tarragon. Pulse process additional 4 or 5 times. Refrigerate until ready to serve.