OK, maybe the mod will allow a question for one of the experts to comment on or answer, on a point that seems important but so far overlooked (at least here).
We have transponder/comms cut off, with some doubt as to whether that was "deliberate" or the result of some electro-mechanical failure/s.
We have a somewhat suspect "alright good nite" hand-over AFTER that.
So MAYBE that is telling. But what about: ATC Vietnam hears hand-over transmission.
Does he also see or note that this a/c handing-over is from a FL he does not see because the transponder is off... would he note that and question that??
(as being alarming to get the hand-over in such circumstances)