Bring a large pot of water to a boil; generously season with salt. Cook pasta, stirring occasionally, until very past al dente, about 2 minutes more than package instructions. Drain.
Meanwhile, in a large bowl, whisk mayonnaise, vinegar, granulated sugar, pepper, and ½ teaspoon salt. Using a rubber spatula, fold in pasta, onion, carrots, and celery. Cover and refrigerate at least 2 hours or up to overnight.
Let stand at room temperature 20 minutes before serving.
