It will only be a matter of time before the country experiences a huge reshuffle in its aviation sector IMHO. The market is not there for so many airlines and also to count the foreign competition.
Among the startups, I'd be worried the most about Bamboo. They seem to have such grand plans for a start-up and it feels like they want to overexpand too quickly. At least this one has the backing of the Vingroup, which is stable financially wise (they even just launched Vinfast, Vietnam's first native car manufacturer) so it might have a chance. But yeah, in the future, I can't seem more than three major players (or two if you count VN and Jetstar Pacific as one); I mean the Philippines with a similar population and economy only really has three major players with two dominant ones, so Vietnam will probably end up in a similar situation sooner than later.