Boil the pasta until very soft (about 1-2 minutes past the package directions). Drain well.
While pasta is still warm, toss it with vinegar. Let it absorb and cool for 10-15 minutes.
Once the pasta is completely cool, add shredded carrots and green onions (if using).
Mix mayonnaise, milk, and sugar (if using) in a separate bowl. Mix it into the pasta and stir until fully coated.
Season with salt and pepper to taste, then refrigerate. Chill for at least 4 hours or overnight for best results.
