In a large bowl, whisk together olive oil, red wine vinegar, garlic and Italian seasoning. Add the onions, mushrooms, peppers, zucchini and tomato and toss gently to combine. Marinate for at least 30 minutes or overnight.
Using an alternating pattern, skewer the marinated vegetables onto the skewers. You should end up with about 12 kabobs.
Preheat a grill pan or gas grill over medium heat (350°F). Brush the grates or grill pan with a little olive oil. Season the kabobs with salt and pepper, then brush them with more of the olive oil and vinegar mixture. Grill (in batches if necessary) for about 6 minutes on each side, or until the vegetables are tender and have visible grill marks.
Brush any remaining olive oil mixture on top of the kabobs before serving.
Storage: Store leftovers in the fridge for up to 4 days in an airtight container. Freeze grilled veggies up to 2 months. Tips:Don't skip the marinade time. Getting a head start on making your grilled veggie skewers should include allowing the cut veggies to soak in the marinade for at least 30 minutes, or up to overnight. This allows all the flavors to meld together and cling to the veggies. The additional brushing of the vegetable kabob marinade ensures you get all the flavor while they cook.Soak wooden skewers. This is an optional step but a great way to make sure your skewers won't burn while grilling. Just a quick 10 minute soak while you chop the vegetables!Oil the grates. This should always be the first step when grilling and the best way to make sure food doesn't stick to the grill. Same with a grill pan, be sure to preheat the pan and oil it before adding the veggie kabobs.Cut vegetables to similar sizes. Each of the veggies on your kabobs cook differently so having all them the same size helps to make sure they cook evenly and make for a hearty variety of veggies on each kabob.