The most likely reason we know so little
Anyone wondering why there is so little information on this lost a/c should look to the recent admission by Malaysian civil aviation chief "There are some things that I can tell you and some things that I can't."
In this part of the world, which I know well, one of the key priorities for any investigation is to ensure that national loss of face is minimized. This means that if the Malaysians have a better idea of where to to look they are going to do their utmost to find the a/c before they share any of this information with the public, let alone the other countries participating in the search. It would be a huge embarrassment to them if say a Chinese frigate were to find the plane first. If there is any hint of error on behalf of the Malaysian flight crew, say from an ACARS message, then I am sure this will not be shared until the a/c is found.
It is all very well to try to dream up scenarios where a plane vanishes without visible debris, but the most likely explanation to my mind is that the SAR are not coordinated internationally.
The same goes for the Vietnamese and Chinese. If they have promising leads, then there is a strong possibility they will not share this information, at least not in a timely manner. Furthermore, if something is found, the information must flow all the way to the top before it is shared, even within an individual agency.