Judith

I haven't done any research on this but often there will usually be a book called something like, "How to find your ancestors in such & such Country"
Call your local library's geneological room and ask what is available there. Call any historical societies in your area and also see if there is some sort of Vietnamese organization or society in your area.
Sometime today I will see what I can find on World Cat for you as to book titles that might help. When records are lost it can sometimes be impossible, but do not give up without trying. It could take several years.
When my first husband was in Vietnam, one of his jobs was to place babies of his men in orphanages. Often the mothers never told their families they had a baby. These were women that had left the farms to find work in the cities.
Also, once records are destroyed there is almost no way to find anyone. In the US the 1890 census was destroyed (fire in the COmmerce Bldg) and so were many military files that were housed in the St Louis area ( from a fire c. 1970) These are gone forever.