Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 10 minutes; rinse under cold water and drain. Transfer to a large bowl.

Pour Italian dressing and vinegar over spaghetti; mix well. Add Parmesan cheese, diced tomatoes, onion, pepperoni, mushrooms, olives, and salad seasoning mix; stir until well combined.

Serve at room temperature, or refrigerate, covered, for about 3 hours, or marinate 8 hours to overnight.