The Best Shish Kebabs

Name: The Best Shish Kebabs
Ingredients: Nonstick cooking spray; 1/2 cup red wine vinegar; 4 scallions, finely chopped; 2 tablespoons Dijon; 2 large cloves garlic, finely grated; 2 teaspoons granulated sugar; 2 teaspoons Italian seasoning; 1/2 teaspoon red pepper flakes; Kosher salt and freshly ground black pepper; 1/2 cup olive oil; 1 pound beef tenderloin, cut into 1- to 1 1/2-inch pieces (about 20 pieces); 1 medium red bell pepper, cut into 1-inch pieces (about 16 pieces); 1 medium yellow bell pepper, cut into 1-inch pieces (about 16 pieces); 2 medium red onions, root left intact, cut into 16 wedges; 1 large zucchini, cut into 1/2-inch thick rounds (about 16 pieces)