Here is what I heard from a friend of a friend. She says she's just looking into adoption from Vietnam and is frustrated in that when one person claims to have had a bad experience with an agency, others swear by that same agency. For example, I've heard bad things about IAAP, yet many love that agency. Anyway, here are some places to research.
Jess
VORF is mostly considered good as is PLAN.
You will hear both good and bad but they seem to be ethical, maybe
lacking in hand-holding/customer contact, but pretty good. IAAP is
very good, and the biggies - Holt, Dillon, CHopeI, PearlSBuck
Foundation, etc are all ethical and good but may be a bit pricy.
Make sure the agency is really licensed in Vietnam by checking the
embassy web page (don't remember the link) and not 'umbrellaing'
under someone else's license (big NO-NO). Smaller agencies are
cheaper but also more likely to cut corners in the in-country
support areas.