There are two types of understanding you have to separate. The first one is acoustic understanding. Of course, North Vietnamese and South Vietnamese speakers can understand each other when they actively hear and talk like humans.
The second one is emotional and mental understanding. This one relates to emotional intelligence. Not everyone has highly emotional intelligence or tries to understand others kindly. Therefore, in General, North Vietnamese and South Vietnamese don't understand each other mentally and emotionally due to significantly different local customs and history. Many North Vietnamese consider the South Vietnamese to be sons and daughters of boat people and slaves of American invaders in the Vietnam war, or even cheap prostitutes of Americans. The North Vietnamese say that the South Vietnamese spend all money they earn for eating and playing without thinking of the future. On the other hand the South Vietnamese hate North Vietnamese and consider them to be communists, who are just loyal to China! The South Vietnamese look down on the North Vietnamese. I have a girl friend in the Northern province Hưng Yên, she said that her parents never allow her to marry a Southern man.
Vietnam is a splitted country. When the Covid-19 rises in the South, the North Vietnamese brutally blame the South Vietnamese on expanding of the pandemic. The Northern people say that the Southern people cannot be governed and are guilty and stupid.
Thus, the North Vietnamese and South Vietnamese don't understand each other in sense of a united folk. This is a failure of Vietnam.