Cook macaroni according to package directions. Rinse under cold water and drain well.
Sprinkle vinegar over the noodles and toss with shredded carrot and onion. Chill 15 minutes.
Whisk mayo, milk, and sugar until smooth. Fold gently into pasta.
Season with salt and pepper. Cover and refrigerate at least 4 hours (preferably overnight).
Stir before serving. Add milk if needed to loosen.
