Cut potatoes into 1" cubes. Place in a large pot of water and boil for 10 minutes. Drain water, add fresh water and cook until tender.
Cut bell peppers into bite-sized pieces, cut mushrooms in half, slice zucchini and mince garlic.
Add the bell peppers, mushrooms, zucchini, garlic, olive oil and rosemary to potatoes. Mix well.
Spread vegetables on a broiler pan, sprinkle with black pepper, and broil for 12 to 15 minutes, until vegetables are slightly browned at the edges. Stir a few times during cooking.
Add vegetables back to the large bowl and toss with the balsamic vinegar. Serve hot or at room temperature.