- however, at an international boundary it would be quite unusual, though not a sackable offence, for the pilots to get away with such a casual hand-off from Malaysian ATC without at least their callsign
Given the informality, they should really have been asked by ATC for a "readback" of their instructions to "Contact Vietnam on frequency xxx.x" (or something like that) to be sure that MH370 was transferring to the correct ATC in the chain.
I would comment that not everyone, and especially not younger pilots, shares the PPRuNe obsession with textbook R/T procedures, particularly on a quiet, familiar VHF channel.
At worse, if you got the wrong freq, you'd flip the switch back on the radio panel and ask the last controller again. I fly though the VVTS FIR from time to time, seems like the southern frequency is 133.05 from memory. If I got it wrong, I could also try to look it up on these new fangled electronic charts (not trivial with the new Jepp software in my opinion), listen on guard, call on 8942 HF or whatever. But, in all probability, the frequency would be easy to find if I botched it on the handoff in the middle of a moonless night.
Also, media reports of an aircraft transmission always seem to get things garbled. Unlike, say, the NTSB which has several pilots and non-pilots carefully vet the CVR transcript, these press reports may be sound bites repeated from an early briefing to government ministers. Once these 'details' are put out to the families and later announced at a media conference, it is hard to tell whether subsequent differing versions are corrections or more misunderstanding of the original transmissions.