Bring 4 quarts of water to a boil. Add macaroni and 1 T kosher salt. Boil for 15 minutes until macaroni is VERY SOFT. Drain and return macaroni to the pot. Add vinegar and toss until absorbed. Let cool for 10 minutes.
Whisk together 1 1/2 C milk, 1 C mayonnaise, brown sugar, salt, and seasoned pepper. Pour over macaroni after it has cooled for 10 minutes.
Let the macaroni cool completed. Once cooled, add 1/2 C milk, 1 C mayonnaise, green onion, celery, and carrots. Stir to combine. Refrigerate for at least a hour, preferably overnight.