This is a good question and it's something that has been reexamined among scholars lately. The emerging consensus is re-imagining the political actors that took involved on the Vietnamese side that casts the Vietnamese conflict as one that is driven through domestic politics of nationalism, decolonization, and multiple religious/political/regional fault lines.

I think the best introduction to this new reexamination is detailed in Edward Miller and Tuong Vo's The Vietnam War as a Vietnamese War: Agency and Society in the Study of the Second Indochina War.

Regarding the question of a civil war, the article states:

For example, many scholars have portrayed the Vietnam War as a Cold War proxy conflict while neglecting its civil war aspects. By contrast, Shawn McHale demonstrates that a civil war among Vietnamese (or, more precisely, an overlapping collection of conflicts among ethnic, religious, and political groups) was already underway by 1945 in the Mekong Delta.

More broadly, the article makes a short case for the examination of the war through a Vietnamese lens which gives agency to domestic actors who often sought the aid of outside powers for their own ends and visions of the country. As alluded to in that above quote, it's a bit of a mixture of both but more of a civil war than it was previously believed to be.

Regarding the south, the region was pocked with an enormous amount of religious, linguistic, ethnic, and political diversity stemming from its frontier-history, isolating geographic features, lack of centralized government, and greater diversity. This localized power structure promoted a unique form of syncretic Buddhism which had to incorporate local diverse superstitions. There was also a noticeable lack of Confucian influence in the south due to fewer footholds of elites in frontier villages. This syncretic and diverse nature would lead to diverse religious millenarian movements that operated as political actors throughout the colonial period. This included the culmination of the formation of the Cao Dai in 1926, the Hoa Hao in 1939, and the Binh Xuyen in the early 1920s which would own up to 1/3rd of the southern territory in 1954 and were generally more popular there than more western-influenced movements such as the ICP (Communists) or VNQDD (Vietnamese party modeled on the Kuomintang), who had stronger footholds in the more homogenous north. So when Vietnam experienced the break it did in 1945, the south balkanized along these politico-religious lines. (See: Jessica Chapman's Cauldron of Resistance for the genesis of these southern sects)

So already you are starting to see massive fault lines that stem from Vietnam's unique history that would undermine any one group from creating a foothold in the country before the formal war even began.

In this same sense, Vietnam's most famous leader, Ngo Dinh Diem, is also a product of a diverse and syncretic past. Edward Miller's Misalliance is a great look at the US-Diem relationship and the unique nation-building programs that took place. Far from a reactionary old mandarin, Diem took inspiration for his nationalistic nation-building programs from his own background, including a revolutionary vision for Vietnam based on the tenets of Personalism, though informed by southern and central Vietnamese traditions. Not a simple puppet, he had a very specific view for decolonization, nationalism, democracy (as he defined it), mutualism, and nation-building that all stemmed from the unique historical background of southern Vietnam.

As such, we can surmise that the diversity of political, religious, and philosophical views created many issues that would make Vietnam such a volatile region upon such a chaotic decolonization project. Its been somewhat recently that this side of the Vietnam War has been explained and their actors given full agency. While the Cold War certainly can't be ignored in the equation of Vietnam, the often overlooked and very complicated domestic policies posed the country for civil conflict along many lines.

Good reading on this:

Edward Miller's Misalliance

Christopher Gascha's Vietnam: A New History

Jessica Chapman's Cauldron of Resistance

David Marr's Vietnam: State, War, and Revolution (1945-1946)

Kevin Li's Partisan to Sovereign: The Making of the Bình Xuyên in Southern Vietnam

Edward Miller and Tuong Vo's Vietnam as a Vietnamese War

Frederik Logevall's Embers of War