Search tip: Keep your search general.
Begin with a keyword search, for example
immigra* and vietnam*
vietnamese americans and politic*
(Note that the * is a truncation symbol, e.g. immigra* returns results for immigrant, immigrants, immigration, etc.)
When you find a record for a book that looks useful, take a look at the subject headings. These subject headings are clickable links that run a new search. Searching by these subject headings may help find resources that don't turn up with an initial keyword search. They might also give you ideas of other search terms to use in your keyword search.
Also try searches of
[ethnic group name] -- Biography
[ethnic group name] -- History
[ethnic group name] -- Social conditions
[country of origin] -- Emigration and immigration -- History