Whisk together milk, mayonnaise, sugar, pepper, and 1/2 teaspoon salt in a bowl.

Reserve 1 cup of the dressing for finishing the salad. Refrigerate both portions.

In a large pot, bring 4 quarts of water to a boil.

Add 1 tablespoon of salt and the pasta, cooking until very soft, about 15 minutes.

Drain pasta in a colander and shake to remove excess water.

Transfer pasta to a large bowl, add vinegar, and toss until absorbed. Let cool for 10 minutes.

Stir the larger portion of dressing into the pasta until fully combined.

Add scallions, carrot, and celery, mixing well.

Cover and refrigerate until thoroughly chilled, about 1 hour.

Mix in the reserved 1 cup of dressing.

Adjust seasoning with salt and pepper to taste. Serve chilled.