Boil macaroni until very soft (1-2 minutes past al dente). Drain and return to pot.
Stir in apple cider vinegar while pasta is still hot. Let sit for 10 minutes.
In a bowl, whisk together 1 cup mayo, ¼ cup milk, and sugar.
Add to macaroni and mix thoroughly. Let cool for 20 minutes.
Fold in remaining mayo, milk, and any desired mix-ins. Season with salt and pepper.
Cover and refrigerate for at least 2 hours before serving.