Alternate beef, mushrooms, bell pepper, onion and zucchini on skewers*. Place in a shallow baking dish in a single layer.
In a small bowl, whisk mayonnaise, salad dressing mix, vinegar and water. Reserve 1/2 cup in a separate bowl and refrigerate. Pour remaining mixture over skewers; turning until evenly coated. Refrigerate for 1 hour or up to 4.
Preheat grill.
Lightly oil grill or spray with vegetable cooking spray. Remove skewers from marinade and place on grill; discard marinade. Cook 10 to 15 minutes, or until desired doneness is reached, turning once. Serve immediately with reserved mayonnaise sauce.
*If using bamboo skewers, soak in water for an hour before using to keep them from scorching on the grill.
Servings: 6