The division of Vietnam (and forcing Vietnam to stay divided) was entirely the fault of France, then the United States.
Except...Annam and Tonkin had been separate cultural/political entities for centuries and their merger dates no further back than the beginning of the 19th century? And that they had little more in common with each other than either did with Laos or Cambodia, the other parts of French Indochina? Arguably the unification of Vietnam is the aberration that owed its roots to French colonial administration of Annam and Tonkin as one unit, and the abberation is not its division in the early days of the Cold War.