Fill a large pot with water, add a generous pinch of salt, and bring to a rolling boil. Add the rotini and cook until al dente, about 8-10 minutes.

Drain the pasta and rinse under cold water immediately to stop cooking and remove excess starch. Shake well or spread on a tray to cool faster.

While the pasta cools, dice the red onion, cucumber, and yellow bell pepper into bite-sized pieces. Halve the grape tomatoes.

In a medium bowl, whisk together the Italian vinaigrette, mayonnaise, white sugar, Cajun seasoning, and ground black pepper until smooth and fully combined. Taste and adjust seasoning.

In a large mixing bowl, combine the cooled pasta and prepared vegetables. Pour the dressing over the salad and toss gently to coat every piece.

Cover and chill in the refrigerator for at least 1 hour to let the flavors meld.

Serve cold, garnished with fresh basil or parsley and a sprinkle of shaved Parmesan if desired.