An important clue is that comms were lost within one hour of departure. The aircraft would have been established enroute at its assigned level within this time frame.
A further fact is that no debris has been sighted so far.
Had the flight crossed the FIR and handed over to the Vietnamese or Chinese, we are not informed. All we know is some garbled information of an attempt to relay by the Vietnamese.
The abscence of floating debris would reduce the likelihood of a catastrophic inflight breakup. It is inconceivable that a large aircraft such as the 777 disintegrating at high altitude would not shed some light weight debris which would remain afloat.
The facts known to date would appear to suggest that the aircaft ditched in the sea in a fairly intact state.