Arrange the vegetables in a single layer on a tray or work surface. Lightly spray both sides with olive oil cooking spray. Preheat a grill or grill pan over a medium-high heat. Grill the vegetables in batches until both sides are nicely charred and the vegetables are just tender, about 10 minutes for the pepper, about 8 minutes for the eggplant and onion, and about 6 minutes for the zucchini. When the vegetables are cool enough to handle, cut into 1/2-inch pieces and transfer to a large bowl.
In a small bowl, whisk together the olive oil, vinegar and oregano. Pour over the vegetables and toss. Season, to taste, with salt and pepper. Gently stir in the tomatoes and mint. Divide the salad between 4 plates and sprinkle with the feta.