Most likely scenarios are a fire on board that got out of hand, a rapid decompression or fuel issue (especially from Beijing). Out if those both the fuel issue and fire would have alerted air traffic a pan would have been declared or if more serious a mayday. Yet it seems to have dropped off the map with no warning.
Crashed 153 miles South of Phu Quoc island.
Desperate for the families involved. Doesn't appear to be a lot of official news available yet other than it's missing. B777 safety record is astonishingly good (no fatalities in 20 years until San Fran incident last year where 3 died).
2 people on the list of the missing are safe, they had passports stolen over the past 18 months.