Cook the macaroni is salted boiling water. Overcook them just a bit, so they'll absorb flavors better. Drain and let cool for about 10 minutes, stirring occassionally, so they don't stick together.
While the pasta is cooking, microwave the vinegar and sugar for 1 minute to dissolve the sugar.
In a large bowl, mix together the Miracle Whip, 1 cup mayonnaise, milk and vinegar-sugar mixture.
Fold in the macaroni, carrots and onions. Season with salt.
Chill for at least 1 hour. Right before serving, stir in the remaining 1/2 cup of mayonnaise.
Notes
If you don't like Miracle Whip, feel free to use all mayo.