Vietnam airline is high cost carrier and is the best carrier in vietnam. it has the quality is pretty good, very comfortable( the plane is big and wide) but staffs in plane are not enthusiastic and friendly and especially, they show contempt for the vietnamese. perhaps they only priority foreigners.
jestar and vietjet are low cost carrier, two of carrier are competitors together about the price. but planes of them are smaller, older. and staffs are like above. the quality is normal, especially, delaying and canceling are often.
so i think if you need higher accuracy. i recommend you should choose vietnam airline