The taxation thing with VNA is a very hot topic and it's way too complex to start getting into on this forum
To make a very long story very short, historically the expat pilots were not registered as individuals with the Vietnamese tax office, the airline dealt with our tax obligations through some other internal mechanism within the government
That all changed about 18 months ago and now we are all either finished, or still going through the process, of getting individually legally registered with the Viet tax man.
People have, in the past, had issues with double taxation on return to their home countries, because the required paperwork was not available from the VN gov't.
The agencies say, that this should be less of a problem under the new system, and we should in the future be able to get these tax certificates from the VN government. However, it's a work in progress, as this is the first year we've spent under the new system.
As always, I would really advise anyone who thinks they might be affected by double taxation issues, to get professional advice and don't trust the opinion of anyone on the internet