Whisk together the vinegar with some salt and pepper in a medium bowl. Slowly whisk in the oil until emulsified. Place the eggplant, bell peppers, and zucchini in a large shallow baking dish, add half the vinaigrette, cover and let marinate for 30 minutes at room temperature.
Preheat a grill to medium-high heat.
Bring a large pot of water to a boil, add 2 tablespoons kosher salt and the couscous and cook until al dente, about 8 minutes. Drain well and rinse quickly with cool water, drain again and transfer to a bowl.
Remove the vegetables from the marinade, season with salt and pepper and grill until just cooked through, 2 to 3 minutes per side. Transfer the vegetables to a cutting board and coarsely chop. Add the grilled vegetables, tomatoes, basil and oregano to the couscous. Add the remaining vinaigrette, season with salt and pepper and toss. Top with the mozzarella and parsley leaves.