Place the couscous in a pot, pour 1 3/4 cups of the boiling water over it, cover, and set aside. Place the sun-dried tomatoes and 3/4 cup of boiling water in a covered heatproof bowl.
While the tomatoes soften, whisk together the garlic, oil, dill, lemon juice, and vinegar; set aside. Uncover the couscous and fluff it with fork.
Blanch the carrots, zucchini, and bell peppers in boiling water until just tender. Drain well. Drain and chop the sun-dried tomatoes.
In a large bowl, combine the couscous, sun-dried tomatoes, blanched vegetables, red onions, celery, and dressing. Add salt and pepper. Chill for at least an hour. Add the diced tomatoes just before serving. Top with parsley or dill sprigs.