The hand over on this route is usually KL to Singapore on VHF followed by a short period on Singapore HF/CPDLC followed by a hand off to Saigon on VHF. The communications and ATC are, in my experience (25 years) usually excellent.
Singapore Radio on HF 8942 is one of the better in the region.
Of course, we don't usually keep a listening watch on HF as mostly we use HF Selcal. However, any distress call on VHF 121.5 would have been heard by any aircraft in the area.
Even at that time of night there are dozens of aircraft on that airway. If an emergency transmission was made it must have been heard by someone!
An absolutely catastrophic flight deck event IMHO.