Both Vietnam and North Korea are Asian communist states in the Soviet bloc, yet both evolved differently.
Why is it that Ho Chi Minh never managed to rose to the megalomaniacal status of his North Korean counterpart Kim Il-sung, who turned his state into a de-facto hereditary kingdom?
Vietnam, while officially communist, now opens its markets and sort of practices some form of capitalism, not unlike China. (Perhaps the same can be said for Laos, Vietnam's neighbour?) Whereas North Korea is still the paranoid state that Kim Il-sung left it when he died two decades ago.
Cuba has Fidel Castro, yet thogh Castro has handed power over to his brother Raul, the Castros never seem to have achieved the cult status of the Kims in North Korea either. And now like Vietnam, Cuba has also normalized ties with the United States.
For that matter, how is that that other East European states in the Soviet bloc escape the fate of becoming just like North Korea? (Maybe Nicolae Ceausescu of Romania came closest?)
Cult of personality and the late socialist transition to a market economy are concurrent phenomenon. Vietnam very much was/is a cult of personality state. Since the 1940s the Vietnamese communist party has built and enforced a cult of personality around Ho Chi Minh that forces all students to study his 'thought' in school, pay reverence to him as though he was a familial ancestor, and strictly forbids any criticism or information construed as unfavorable to Ho.
Part of the cult was that Ho Chi Minh had no wife -- he did in fact have a couple wives -- so he could not have a hereditary successor. The party leaders enforced that image, going so far as to prevent Ho from openly wedding his last wife in the 1960s (and if you believe rumors among some powerful dissidents who lived in Hanoi, the party staged a car accident that killed Ho's wife to prevent him from going public with his marriage). Moreover, it was Le Duan who held true power at this time, so he had no need to hand power over to a non-existent heir of Ho Chi Minh. All he needed was to wield the already powerful cult of Ho.
But that political issue is separate from why Vietnam pursued economic liberalization in the 1980s, which seems to be the question you're more interested in. And since North Korea is the outlier here (as China and Vietnam pursued economic changes at the same time in the 1980s) you may have more success asking why it is that North Korea never pursued an economic course similar to China, Vietnam, and other socialist states.