Prepare this deconstructed sandwich at night or in the morning, and store each part separately. At lunchtime, or dinnertime, simply arrange and serve.
Preheat oven to 400 degrees. Line a rimmed baking sheet with aluminum foil. Lay bacon in a single layer on prepared sheet; bake until it begins to render its fat, about 10 minutes. Turn bacon over, and slide to one side of sheet. Season chicken with salt and pepper, and place on baking sheet. Bake until bacon is browned and crisp and chicken is cooked through, 15 to 20 minutes.
Transfer bacon to a paper-towel-lined plate, and let drain; crumble. Let chicken cool completely. Meanwhile, in a large bowl, combine tomatoes, onion, celery, celery leaves (if using), mayonnaise, and vinegar.
Cut chicken into 1/2-inch chunks, and add to bowl. Season with salt and pepper, and toss to coat. Serve chicken salad over lettuce leaves, and sprinkle with bacon.