Was The Plane Shot Down By A Vietnamese Missile?
Obviously I do not know, however the fact is that this is all too possible because of the tensions engendered by China. Indeed a similar event recently occurred off of North Korea.
The plane took off from Malaysia and was first reported as missing on the west coast of Vietnam. Now the evidence is that the airliner disappeared over the South China Sea, an area that China’s imperialist policy claims as its own even though the Sea extends hundreds of miles south of China and is bordered not by China but by Vietnam, Philippines, and Malaysia … all of whom have better claims than the world power far at the North end of this Sea.
The Chinese navy has world class aspirations but for now its activities focus on the Taiwan Strait and the Sea that is named for China. “Fishing boats” land “civilians” on these islands much as “civilians” in Russian uniforms are 
liberating Ukraine.
What do you imagine Vietnam doing to defend its property? Presumably, the small Vietnamese air force prioritizes control of its air apace, That must mean that suspicious objects .. like a plane that turns back toward Vietnam while over the South China Sea …. are all too likely to be targeted by eager, young Vietnamese pilots.
We may never know.
I suspect our government knows where the plane is, and the search is only a pretense to confuse the hijackers while a military operation is planned.
From what has leaked to the media, we now know the transponders were turned off, the plane changed course and flew low to avoid detection, and kept flying for another four hours after contact was lost. That puts the plane 2,000 miles away from the search area, roughly the distance to Iran.
This now looks like a job for Delta Force.