I just completed Vietnam trip. My recommendation is to go for revenue ticket on Vietjet for Vietnam rather than burning your air miles
I would absolutely not recommend Vietjet to anyone, not even to people I hate.
I can say with certainty it's among the worst airlines in Southeast Asia. If things go right, they're fine if you're okay with budget airlines and the risk that comes with them.
Beware of overweight bags and baggage charges because it might cost you as much or more than the ticket itself. They are pretty strict when it comes to it.
Customer service is non existent as well.
But the most important bit: they are among the most unreliable airlines in the region with very high delay and cancellation rates. The way they operate and schedule flights makes them very prone to have operational issues.
You'll likely end up waiting days for the next flight or paying out of pocket for a very expensive last minute flight if your flight is cancelled. Delays are fairly common so take that in consideration when booking hotels or transport.
If you are okay with those risks or have the flexibility in exchange for very cheap tickets, go for it. For connecting flights to third countries I'd absolutely avoid them at all costs.
There's a reason why sometimes they're half the price of other airlines or even cheaper, and you get what you paid for.
If you're gonna do it, make sure you get the best travel insurance you can get, it will pay off!