Bring a large pot of salted water to a roiling boil. Add macaroni and cook until very soft, 10 to 12 minutes. Drain, return the pasta to the pot and immediately toss with vinegar. Let cool ten minutes.
In a large bowl, whisk together 1 cup of the mayonnaise, 1 1/2 cups of the whole milk, the brown sugar, salt, and pepper.
Once the macaroni has cooled for 10 minutes, toss with the mixed sauce and let cool to room temperature.
Once cool, fold in the scallion, carrot, and celery. Whisk together the remaining 1 cup mayonnaise and 1/2 cup whole milk and pour over macaroni salad. Toss well to coat. Taste and add more salt or pepper as needed.
Chill in the refrigerator until cold.