I researched SE Asia somewhat. I also agree that VietJet is, overall, better than Vietnam Airlines. pprune.org posters are not liking Vietnam Airlines at the present time.
A pilot I know just got small Airbus DEC, with no airbus experience, for Jetstar Pacific, based in SGN. I had never heard of them (they are one of the 5? Jetstar airlines). I was surprised they were hiring DEC with NO time in type.
Most people I have talked to said SGN is either better than Hanoi, or better for expats. If you want to get around efficiently in either city, a scooter is a necessity. I think that Hanoi would be cheaper living costs than SGN.