Heat a grill to 400-450 degrees.
Drizzle the vegetables with olive oil and gently rub to coat both sides.
Sprinkle with salt, black pepper, and garlic powder.
Skewer the mushrooms and slice the other vegetables.
Place the red onions and bell peppers on the grill first for 3 minutes. Then place the remaining vegetables on the grill and close the lid, cooking for 3-4 minutes.
Flip the vegetables, close the lid, and cook for an additional 3-4 minutes.
Remove from the grill and drizzle with balsamic vinegar.