Lac Viet is a growing company with various interests related to computing. They have a great dictionary which you can purchase at their site here. Unfortunately if you don’t read Vietnamese their site won’t help much because the English version is perpetually under construction. If you are in Vietnam already you can likely purchase the PC version of their dictionary (MTD EVA) from a local software shop. They also have versions for other operating systems.
Lac Viet also has an online dictionary which you may find helpful. It is really geared toward the Vietnamese speaker working to learn English, but you can search from either language. There are also some online translation tools available. The online dictionary is here.