Here is my super quick answer:

Considering what I got from Vietnamese media, during the Vietnam war, Cuba, which was under Fidel Castro’s regime, gave Vietnam troops the assistance which was believed to be “valuable”, “timely” and “effective”. The country even celebrated the day of Vietnam’s victory as they had hailed: “For Vietnam, we are ready to devote our blood.” Not to mention both are socialist countries and they have remained close-knit relations for more than 50 years.

So I guess it’s rather reasonable for Vietnam to hold a national day of mourning for Fidel Castro in return.