Many Viet Kieu return to Vietnam for many purposes (doing business, working, traveling, visiting their family, studying, or just to know where they come from, etc). No matter what reasons, when the learning inspiration inside them becomes strong enough, they decide to learn their mother tongue.
To me, Viet Kieu is quite a special type of students who need more specifically designed curriculum for Vietnamese learning. Although currently there are not wide variety of sources of learning Vietnamese language, some books on the market are somehow useful depending on what you need. But the materials for Viet Kieu are not much, and if there are any, they are not very instructional.
These are the reasons why Viet Kieu need a different teaching approach: