Pontius :
the diagram above shows the exit from the airway to the right followed by a left hand turn back through the airway
There is no " airway" anymore at this altitude , there are only tracks with no width.
Blake 777 : What the Vietnamese military saw in real time we do not know. . Afterwards , looking at the tapes maybe. For Vietnam civil ATC , the last positive ID/position stopped when the transponder stopped. The left turn shown on the map was probably not seen ( or detected ) by them in real time .
Maps shown 2 months after the event can be very precise when dozen of people have been sudying for hours tons of tape recordings from various sources , but what controllers saw in real time , and were expecting is not what we see on those maps.
So saying they should have done this or that based on those maps is irrelevant and futile.. .