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, and salt and pepper to taste (I like 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. Cover and refrigerate at least 1 hour.
Apple cider vinegar: Adding cider vinegar to the cooked noodles to make them more absorbent so they hold the flavors of the dressing better. If you don't love vinegar or feel unsure, omit completely or start with 1 tablespoon. Rice wine vinegar can also be used.
Scallions: One white or yellow onion, grated on a box grater, may be substituted for the scallions.
Yield: This recipe makes 12 (1-cup) servings.
Storage: Store leftovers in the refrigerator for up to 4 days.
Make ahead: Make this macaroni salad 1 day in advance. Stir to recombine before serving.
Dry: If the pasta salad seems dry, stir in warm water, 1 tablespoon spoon at a time, to loosen.
More mix-ins: Some cooks swear by adding green olives, chopped hard-boiled egg, finely chopped boiled potatoes, watercress, pickle relish, frozen peas, or even canned tuna.