Short answers, pulled from the story.
North Vietnam was proclaimed on the 2nd of September 1945 as the Democratic Republic of Vietnam, with its sovereignty internationally recognized in July 1954 following the Geneva Conference. It ceased to exist on the 2nd of July 1976, when it merged with South Vietnam to form the Socialist Republic of Vietnam.
Hồ Chí Minh, leader of the Việt Minh Front, declared independence and became the first leader of the Democratic Republic of Vietnam in 1945. Lê Duẩn succeeded him as the dominant leader from 1960 onward, overseeing the Vietnam War and the reunification of the country.
The Geneva Accords of 1954 ended the First Indochina War and provisionally divided Vietnam along the 17th parallel into a northern zone under the Democratic Republic of Vietnam and a southern zone under the non-communist State of Vietnam. The accords called for general elections in July 1956 to reunify the country, but those elections never took place.
Estimates of executions during the 1953-1956 land reform vary significantly. Scholar Edwin E. Moise concluded the total was probably on the rough order of 5,000 and almost certainly between 3,000 and 15,000, with a later revised estimate of around 13,500 including suicides following arrest. Economist Vo Nhan Tri reported a central party archive document placing wrongful executions at 15,000.
Sweden became the first Western country to extend full diplomatic recognition to North Vietnam in 1969. By December 1972-49 countries had established diplomatic relations with the Democratic Republic of Vietnam, with France restoring formal ties in 1973.
After the fall of Saigon on the 30th of April 1975, North Vietnam governed South Vietnam through a provisional government for approximately one year. On the 2nd of July 1976, North and South Vietnam were officially reunited as the Socialist Republic of Vietnam, with the new state adopting North Vietnam's constitution, flag, and anthem.