Prepare the vegetables by washing and cutting them into bite-sized pieces.
In a large bowl, combine the zucchini, bell peppers, onion, and mushrooms.
In a small bowl, whisk together balsamic vinegar, olive oil, garlic, Italian herbs, salt, and pepper.
Pour the dressing over the vegetables and toss to coat evenly.
Preheat a grill pan over medium-high heat until hot, or prepare your outdoor grill.
Carefully place the vegetables in a single layer on the grill, working in batches if necessary.
Grill for 3–4 minutes per side until tender and marked with grill lines.
If the vegetables look dry, lightly brush with olive oil and a splash of balsamic vinegar.
Transfer the grilled vegetables to a serving dish and adjust seasoning before serving.
Garnish with fresh herbs, if desired, before serving.