There are three main schools of thought when it comes to items regarding the Vietnam War.

The Orthodox school which largely encompasses general dove like sentiments about the war, usually written by veterans and journalist involved with the conflict itself. The Revisionist school which, although coming to prominence slightly after the conflict, has hawk like stances about the war. And a new emerging school that tends to not regard the stances of these two 'polarizing' schools as not disjoint or mutually exclusive but rather regards the Vietnam war as not being able to characterize the conflict and its proponents in either a good camp or bad camp. It also places the Vietnamese people as a more central player in the narrative that is the Vietnamese war ( this differs from the Orthodox and Revisionist schools that are in large part American soldiers, journalist, scholars, etc writing either during the conflict or in the years after).

Depending on which school the author subscribed to, by and large, reflected what view you'd receive from them on the matter. Orthodox scholars generally view Diem as autocratic and illegitimate. Despite the south being a 'democracy' many elections for positions in the government were rigged including Diem's own election in 1955. Diem had a tendency to appoint family members to prominent position in government, such as his brother Ngo Dinh Nhu as minister of interior; Nhu would play a large part in suppressing political dissent before and during Diem's rise to power. Nhu and his brother Ngo Dinh Thuc, the ArchBishop of Hue, would together play large roles in the suppressing Buddhism despite the large Buddhist presence in the South. Additionally, Diem would suppress public protests and newspapers (at times but never censoring the press entirely) (Some may have seen Diem as an American puppet, but I do not feel qualified enough to expand further on this topic).

In contrast to this, Revisionist scholars believe that Diem was justified in a lot of the actions he had taken in his rise to power as well as during his presidency. South Vietnam was in large part a 'new' country' in that the government was newly formed and in contrast to its traditional monarchical rule and colonial subjugation by France and Japan, had a lot of 'growing pains' in setting up this new government; additionally, as seen in other ex-colonies in Africa and other parts of South East Asia, prominent figures that use both wealth and force to attain political prominence are not uncommon and Diem is arguably one that comes to prominence with, a least publicly, a lesser tendency to use force. Additionally, the autocratic like tendencies of Diem have been attributed to the tough nature of Vietnam at the time, the South especially had a number of factions that vied for power such as Catholics, Buddhist, Cao Dai, Hoa Hao, as well as the fact that Diem has been the target of multiple assassination attempts over the course of his political career.

Vietnamese Centric scholars tend to not view these versions of Diem's legacies as exclusive to one another. However, notably in contrast to orthodox scholars, many view Diem, not as a American puppet, but rather made decisions on his own accord.

Andrew Wiest and Michael J Doidge. Triumph Revisited: Historians Battle for the Vietnam War. 2010. ^^ Chapter 2 in particular discusses this debate and has additional sources from the Orthodox and Revisionist schools of thought as well.