John, I fly that route on a regular basis and communication with ATC is good.
VHF is available for most of the time. The 777 would also have been established on CPDLC data link using ADS-C connection with VVTS. ( unless it was u/s )
There is only a small time spent on HF and even then the HF quality with Singapore is excellent most of the time.
If this happened in Vietnam airspace as posted then they would have been on VHF with Hochiminh and identified on radar.
I'm guessing as we all are but something catastrophic must have happened for the crew not to have said something to someone.