Prepare an outdoor grill, letting the coals burn until they are covered with white ash.
Brush the zucchini, yellow squash, eggplant, onion rings, and bell pepper with a thin coat of olive oil, and season them with salt and pepper.
Grill the zucchini and yellow squash for 3 minutes on each side, turning them over once. Transfer them to a plate and set aside. Grill the eggplant until nicely caramelized and tender, about 4 minutes per side; then transfer it to the plate with the other vegetables. Grill the onions for 4 minutes on each side, and transfer them to the plate. Grill the bell pepper until all sides are charred, about 10 minutes. Put the pepper in a bowl and cover it with plastic wrap. Let it steam in its own heat for 10 to 15 minutes. Remove the pepper from the bowl and peel it. (The charred skin should come right off with the aid of a paring knife.) Cut it open and remove the seeds.
Cut all the vegetables into bite-size pieces and set them aside.
Bring a pot of lightly salted water to a boil over high heat. Add the couscous and cook until it is tender but still firm to the bite, 7 to 8 minutes. Drain, toss with the 2 tablespoons olive oil, and set aside.
In a small bowl, whisk together the lemon juice and garlic. Add the remaining 1/4 cup olive oil. Season with salt and pepper.
In a large serving bowl, toss together the couscous, vegetables, tomatoes, parsley; and basil. Dress carefully with the vinaigrette, toss gently; and season with salt and pepper. Serve.