La Belgique is a restaurant serving French cuisine and ethnic food such as osso busso. They serve 5 kind of couscous: chicken, beef brochette, merguez, lamb chop and royal. Starting at 210,000 VND for the basic one, chicken or merguez, and 285,000 VND for the royal one.
Warda is located inside the Hem 71/7 on Mac Thi Buoi street. With a nice oriental decoration, the restaurant has indoor and outdoor, the cuisine is really great, you can enjoy a couscous for 285,000 VND and even a chicken and olive Tajine for 205,000 VND.
Papagayo sometimes serve couscous on the weekend only, you can check on their Facebook page, they always announce and promote it on it and it costs 260,000 VND per plate.
Similar to a couscous and prepared with the same ingredient, Saffron is also a great place where they serve Moroccan chicken with couscous. See the full menu here.