To roast red peppers, place under broiler or rotate over a gas flame until skin blisters and is completely black on all sides. Place peppers in a paper bag for 15 minutes to cool. Scrape away charred skin, then seed. In a blender or food processor, puree peppers, garlic and vinegar. Scrape into a bowl and set aside. In a large saucepan, bring 4 cups water to a boil. Stir in couscous and margarine, cover and remove from heat. After 10 minutes, stir again to fluff. Add red pepper puree, onion and seasonings to couscous and mix well. Place in a lightly greased ring mold and refrigerate at least an hour. To serve, unmold on a serving plate and fill with vegetables. Combine yogurt and dill in a small bowl and serve as a sauce with the couscous ring.