Cook macaroni in a large saucepan according to package directions, except omit any oil or salt. Add mixed vegetables, zucchini, and sweet pepper during the last 3 minutes of cooking; drain. Return macaroni mixture to the saucepan.
Meanwhile, whisk together evaporated fat-free milk, chicken broth, flour, oregano, garlic powder, pepper, and salt in a medium saucepan. Cook and stir until thickened and bubbly. Add to macaroni mixture; toss to coat. Spray a 13x9x2-inch baking dish or 3-quart rectangular casserole with nonstick cooking spray. Spoon macaroni mixture into dish.
Bake, uncovered, in a 375 degree F oven for 25 minutes. Top with tomato slices and sprinkle with Parmesan cheese. Bake about 5 minutes more or until heated through. Let stand for 5 minutes before serving. Makes 8 servings.