UN International School (UNIS) offers group classes - (http://www.unishanoi.org/pages/community/comed/comed_home.html). There are various language centers around but they offer one-on-one training - unless you pull together a group. Lots of individual tutors available.
My friend is fully experienced in teaching Vietnamese language for expats in Hanoi. Her name is Nhung. If you are interested in, contact her for more information: lekamnhung@yahoo.com