In a small bowl, whisk together the vinegar, mustard and garlic, slowly add the olive oil and whisk until combined. Season with salt and pepper. Pour 1/2 the marinade over the vegetables and let sit at room temperature for 15 minutes. Preheat the grill. Remove the vegetables from the marinade and grill the vegetables until just cooked through. Cut the zucchini and peppers into 1/2-inch pieces, cut the tomatoes in half. Heat the olive oil over medium-high heat, add the couscous and toast until lightly golden brown. Cover the couscous with the hot stock plus hot water and bring to a boil, cook until al dente and drain well. Place in a large serving bowl, add the grilled vegetables and herbs and toss with the remaining vinaigrette. Serve at room temperature.