Im American and live in Vietnam. I fly around SE Asia a lot. Vietnam Airlines is generally quite good. There are lots of budget airliness in SE Asia that are extremely cheap, but you really get what you pay for.
As I think when people on the flight went to take their baggage when the belt-off signal hadn't been turned off, maybe it was people' awareness. Despite not being in too many flights, but I realized it's a normal thing with Vietnamese airlines. Of course this will not happen much in a big one like VNA, but if it was a cheaper one, like VietJet (or VJ), you'll have to accept that. It's quite hard to change people' mind. Another thing with the AVOD, you could ask for help if it didn't work. As far as I know, AVODs on VNA's A350s works pretty well, and many reviewers (including Vietnamese) experienced it - in a normal way. In short, VNA (afaik), is a 4-star airline. It's a good one, and maybe you should try it again, give it another chance ^^. From Vietnam with love, Cảm ơn.