Bring a large pot of salted water to a boil. Cook macaroni until very soft (1–2 minutes past al dente). Hawaiian mac salad isn’t al dente — the pasta should be tender and plump so it soaks up dressing. Drain well. While pasta is still warm, toss with vinegar. Let cool for 10 minutes — this step helps the macaroni absorb flavor.

In a bowl, whisk together mayonnaise, milk, sugar, salt, and pepper until smooth. Fold in cooled macaroni, grated carrots, and onion (if using). Mix well so every piece is coated. Taste and adjust seasoning (add a splash more milk if too thick). Cover and refrigerate at least 2 hours (overnight is best). The flavors meld and the pasta absorbs the creamy dressing. Stir before serving.