Bring a large pot of lightly salted water to a boil. Add the macaroni, and cook until tender, about 8 minutes. Rinse under cold water and drain.
In a large bowl, whisk together the mayo, vinegar, sugar, mustard, salt and pepper. Stir in the red onion, celery, green bell pepper and macaroni. Refrigerate for at least 4 hours before serving, but tastes best after sitting overnight. Serve and enjoy!