Slice eggplant into ½-inch-thick rounds. Soak in a bowl of lightly salted water for 15-20 minutes. Drain and squeeze out the excess moisture.
Slice tomatoes and zucchini into ½-inch-thick rounds.
Stem and seed the bell pepper. Slice the pepper and onion into ½-inch-thick rings.
Heat a grill or grill pan to medium-high heat. Spray the vegetables with zero-calorie nonstick cooking spray on both sides and season with ½ tsp salt and pepper to taste.
Grill the vegetables (in batches if necessary), turning, until soft and marked, about 6 minutes for the tomatoes, 7-8 minutes for the bell pepper, onion, and zucchini, and about 10 minutes for the eggplant. Let cool.
Drizzle the vegetables with vinegar.
Divide the zucchini and eggplant among plates and arrange them in a circle, overlapping slightly. Top with the bell pepper, tomato, and onion. Sprinkle with basil.