Musèe Cernuschi has a substantial Vietnamese art collection:
Vietnamese Collections | Musée Cernuschi
Otherwise, the Choisy triangle (the area between Boulevard de Choisy and Boulevard d'Issy, south of Rue Tolbiac and including the housing projects at Olympiades) in the 13th Arrondissement is known as "Asiatown", although recently this means more and more "Chinese" rather than "Vietnamese".