I've just done a 16 day trip to Vietnam, flew back Sunday morning, 1am flight!
We flew with Vietnam Airlines in economy direct from Heathrow to Hanoi in the Dreamliner.
It was pretty average to be honest. In-flight entertainment was so-so, the usual dubbing of swear words and some scenes removed but the annoying thing was hard-coded Vietnamese subtitles, but there was some pretty recent stuff on it.
Food was ok, you get two options to choose from, Western and Vietnamese with two meals being served. The Western option is pretty bland, and it was the only option for us as we sat at the back! Instant noodles and sandwiches are available on request.
Not stingey with the drink though, and there's a decent selection.
But the annoying thing is that the Vietnamese seem to generally disregard their booked seats and just sit wherever they want. We had people in our seats (we were a party of 5) which we had booked to get a nice selection of aisle seats but the flight attendands didn't seem to willing to move them so we ended up sat elsewhere, which luckily was acceptable.
I plan on going back but will probably fly with Emirates.