According to the Malaysia military and them solely the plane flew west to some of the deepest water in the region 1,000-5,000 feet deep. It might take a decade to to find any debris.
Nothing to see here...move along.
Conversely, if the plane went down in Vietnam waters, trust there won’t be any unofficial investigations.
Trust that the prodacol for shooting down a rogue plane is in place, implementing that takes minutes.
Creating a plausible cover story...well this takes much longer. Hmm...what might that look like in the news short term?
The cover story serves the greater good, who wants to be the first government to order the shoot down an airliner or the entity physically tasked with the shoot down? => Think greater good.
Perhaps, the one thing I see is most of the passengers were Chinese, correct? So, it seems the wrath of China would be what to fear in this situation. China would not be content to see it just swept under the rug.