As an expat in VNA 321 for more than 6 years, I will try to tell you the "FACT", instead of my "personal feeling".
1.Yes, expat FO do get upgrade in 321. I will say, if you are willing to stay in VNA for 3 years (or more) as 321 FO, and you dont have any serious viloation of CAAV, FOM....etc, then your chance of getting the upgrade is 80%. You lose the 20% chance mainly because "they just dont like you".
2.There is no answer to "VNA is good or not to work for". For salary you should ask your agency. Just keep it in mind, the salary has not been improved for more than 6 years, it stays the same from at least 2009. Roster in 321 is a nightmare, this is both a "FACT" and my "personal feeling", and I beleieve more than 95% expat in 321, CAP or FO, will agree with me. Very often, you fly close to, or even over, the maxium duty time, followed by a minimum rest. Very often, you get schedule cnanges. Even on your DO, you will get a call from company and they will ask politely: "I know its your DO, but can you fly today due to ...."
3.If all you are looking for is cheap food/beer/romance...anything fast and cheap, you want nothing else, then you will be happy in VN, as an expat "male" pilot. If you want more, for example: quality art exhibition or dance performance, a big park you can wander around for 30 minutes or have a picnic without worring about the air pollution....then, you will find the options for your free time in VN are very limited.
4.My "happiness level" is VNA is getting lower and lower every year in 321. This is a personal thing, but I believe at least 70% expat in 321 will agree with me. Mainly because the tiresome schedule, and also the way Vietnamese people dealing with things.
5.Agecies are more or less the same, really.
Simply put, if you are dying for a chance of command upgrade within 3~5 years, and you are willing to pay around 50000-60000 USD, including the training and bond and 2~3 months unpaid, then VNA is for you. If you are looking for a long-term career, VNA is not a good option.
And one last thing, keep this in mind please, because you are probably thinking about getting a fast command and leave! You pay for the upgrade by yourself, but still it is possible to fail the upgrade, and you can not get your money back! Keep this in mind please, this is serious! It is not "I am paying for it, so you will let me pass" training. In 2014, at least 2 expat FO fail the 321 upgrade trining.
You have been warned!