Boil a large pot of water, salt it, and cook macaroni until very tender, 10–12 minutes.
Drain well. While hot, sprinkle vinegar over pasta and toss to absorb. Let cool 10 minutes.
Whisk mayonnaise, milk, sugar, pepper, and 1 teaspoon salt in a large bowl until smooth.
Fold in carrot, onion, and celery until evenly distributed.
Add cooled macaroni and stir gently until every piece is coated. Mix in green onions if using.
Cover and chill at least 30 minutes. Taste and adjust salt and pepper before serving.