Cook macaroni according to package directions. Drain well and place macaroni in a large bowl (I like to use a tupperware bowl with a lid so I can easily refrigerate later).
While macaroni is still hot, sprinkle on vinegar and add carrot and onion. Toss together until well combined. Allow to cool for about 10-15 minutes.
In a separate, smaller bowl, whisk together mayo, milk, and sugar. Fold mayo mixture into the macaroni until all the noodles are evenly coated. Add salt and pepper to taste.
Cover and refrigerate at least 4 hours (best if overnight). Gently stir before serving (adding a little more milk if needed, no more than a Tbsp. or two).