In a large stockpot, bring 4 quarts water and 1 tablespoon salt to a boil, Stir in pasta and cook until very soft, about 15 minutes, stirring often.
Drain well, transfer to a large bowl, and stir in vinegar. Cool at least 10 minutes.
Meanwhile, in a small bowl, whisk together milk, mayonnaise, brown sugar, 1/2 teaspoon salt, and 2 teaspoons pepper. Pour over cooled noodles and stir until evenly coated.
Fold in scallions, carrots, and celery and stir to combine. Season to taste with salt and pepper. Cover and refrigerate at least 1 hour.