What would the Malaysian ATC have done?Should have called the...

What would the Malaysian ATC have done?

Should have called the aircraft, of course, and kept the Vietnamese control advised. Either Malaysia or Vietnam should have declared the appropriate phase - Uncertainty, Alert or Distress. Probably starting with Uncertainty and then progressing up the levels as time passed without contact.

I do not know what was actually done but these are very standard procedures - routine stuff.