In a small bowl, combine the mayonnaise, basil, garlic, and lemon juice. Set aside. Coat a grill rack with cooking spray. Preheat the grill. Brush the onion, bell pepper, and zucchini with the olive oil. Sprinkle with salt and pepper.
Place the onion and peppers on the rack and grill for 10 to 12 minutes, turning once, or until the vegetables are well marked and tender. Place the zucchini slices on the rack and cook for 6 to 8 minutes, turning once, or until marked and tender. Bread can also be grilled at this time, if desired. Layer 4 slices of the bread with an onion slice separated into rings, 2 bell pepper slices, 2 zucchini slices, and 2 tomato slices. Spread the remaining 4 slices of bread with the mayonnaise mixture and place on top of the tomatoes. Cut each sandwich in half to serve.