Cook the pasta: Boil macaroni 2 minutes longer than package directions until soft. Drain well.
Add vinegar: Toss hot macaroni with 1 tablespoon vinegar in a large bowl. Let cool for 10 minutes.
Mix veggies: Fold in grated carrots, diced celery, and sliced green onions gently.
Make dressing: Whisk together mayonnaise, remaining vinegar, sugar, milk, salt and pepper until smooth.
Combine: Pour dressing over pasta mixture and fold until everything is evenly coated and creamy.
Chill: Cover and refrigerate at least 1 hour before serving. Overnight is even better!