1. The North Vietnamese government, contrary to common perception, was not a brainless hive mind. Several factions existed, split over questions of relations with South Vietnam, tactics, whether to favor Moscow or Beijing's advice, etc. However, over the course of the 1960s, the power of the state bureaucracy was increasingly concentrated in the hands of one man, Le Duan, the General Secretary of the Communist Party. He was the guy who was really in charge for the time period that is what Americans know as the Vietnam War. Ho Chi Minh, the public face of the regime until 1969, was more or less powerless during this period. (One has to remember that Ho Chi Minh was older than Mao Zedong-by Third World standards in the 1960s, he was positively ancient-and thus not in the best of health in his last years. It is highly likely that Ho's "reply" to Nixon in 1969 was actually written by Le Duan under Ho's name.) Le Duan would launch some pretty nasty periodic purges to strengthen his hold on the government, and as the war continued, North Vietnam's police state nature would become more and more pronounced.

The ironic thing was that Ho, for all his deserved reputation for tenacity, was far more flexible than Le Duan when it came to the "how" of achieving a unified Communist Vietnam. Had Ho actually been in charge, he might have accepted Nixon's peace proposal in May 1969, if not LBJ's in Manila in 1966, provided it ensured the Americans left for good. But Le Duan believed that Ho's decision to trust their allies and negotiate in 1954 was what led to the partition of Vietnam in the first place, simultaneously hammered home by what was happening in Korea around the same time.

And in fairness to Le Duan, what was there to negotiate, in his viewpoint? It was very clear by 1968 that there was a lot of opposition to the war: why trust the foreigners when you can simply hang on and bide your time? The Americans needed to get out and let the Vietnamese settle their scores undisturbed. To insure North Vietnamese unreliance on foreign powers, he worked endlessly-with a significant if not infallible degree of success-throughout the 1960s to minimize Soviet and Chinese leverage over North Vietnam, to the point where he could dictate what happened at the table. It all worked tremendously until Nixon changed the rules of the game, in totem, with Hanoi's patrons in 1971-1972, and even that proved only a temporary setback.

2. After Diem's assassination, South Vietnam went through a cycle of 20-something governments, with the end result being that they only controlled 13% of their territory by 1967. The Viet Cong more or less ruled the countryside-they even levied taxes. Power in Saigon and the other coastal cities ended up largely settling into the hands of three men-Nguyen Van Thieu, Nguyen Cao Ky, and Tran Thien Khiem. There were a lot of intrigues between Thieu and Ky and their sections of the military throughout the Johnson years-often, they fought each other more hard than the VC, to the endless consternation of the Americans. One particularly interesting episode was Ky's secret contacts with the South Koreans, which may have helped reinforce Thieu's decision to not cooperate with LBJ and the Democrats in the fall of 1968. But in the end, the far more down-to-earth Thieu gradually sidelined Ky. By 1968, Khiem was the PM, Thieu the President and far and away the more dominant of the duo.

It is also important to note that, while certainly no paragon of Western-style democracy, in contrast to many of our Asian allies (say, South Korea) at time and certainly the North, by the early 1970s, South Vietnam did have a viable opposition in their political culture and a relatively free press. This included people who favored negotiations with the Viet Cong and a possible role for them in the South Vietnamese government, not least because the Viet Cong was largely comprised (especially in the Johnson years) by fellow Annamese and Southerners. This complicated things endlessly for US-GVN relations.

(One tangentially related thing to point out is that the nature of the Vietnam War changed over time, as well as just what the anti-Thieu intellectuals and politicians in South Vietnam wanted. Being anti-Thieu, advocating negotiations with the VC, and generally wanting an end to the war was not the same thing as a desire to live under totalitarian rule from Hanoi-a distinction that many Western leftists tended to ignore in the early 1970s when it became increasingly clear that it was now PAVN rather than the VC doing the bulk of the fighting. As for the war itself, in 1965, you had a massive insurgency in the countryside and cities carried out by the Viet Cong, who, it is crucial to state, were mostly Southerners who saw themselves as equals to the North, rather than subordinate to them. It has actually been speculated that one of the reasons Hanoi launched the Tet Offensive was to trim the ranks of the VC and assert unquestioned dominance of the struggle in the South. If so, that was exactly what happened: the Viet Cong was a shadow of its former self after Tet, which was one of the reasons South Vietnam's rickety but stabilizing government was able to take back much of the countryside during the Nixon years. By 1972, with the American presence from the country much diminished and about to be removed entirely, you had North Vietnamese tanks crossing the DMZ and starting a conventional, WWII/Korean style offensive-with total military conquest of the South being the goal.)

3. I would recommend the works of Lien-Hang Nguyen and Pierre Asselin, as they got access to the Vietnamese archives (and in the case of the latter, Bulgarian/Russian archives, to see what the relationship between North Vietnam and its allies was like), and unlike many American authors and media personalities, try to emphasize the actual conflict between the two warring Vietnamese states rather than America-centric politics when explaining ultimately why the Vietnam War took the nature it did and why it dragged on for so long. There is an annoying tendency in US historiography (both left and right-wing) to implicitly reduce the Vietnamese to bystanders in their own civil war, often for nakedly political reasons.

To be fair, Third World conflicts during the Cold War were invariably complex in nature, the Indochinese Wars more complex than most, and often require a somewhat detailed knowledge of all the parties involved to fully get the gist of. But the America-centric view of the Vietnam War found all over the American political spectrum isn't just inaccurate, but also somewhat insulting to the Vietnamese-on both sides. It was, after all, the Vietnam War, not the Americans in Vietnam War for a reason: the death toll for the US numbered in the tens of thousands, the death toll for Vietnam was in the millions, with millions more fleeing or being expelled from Vietnam in the aftermath of the war. There was nobody in Vietnam, North or South, who did not lose somebody.

Nguyen is especially good at describing Le Duan, his life, and what he did in power: he was a fascinating figure, not least because unlike many in the Northern leadership, he had "Southern" (Annamese, actually) origins, which helped make him into the militant that he became. Thieu, too, was Annamese, and a Buddhist convert to Catholicism. This forever cast a shadow on his dealings with the predominantly Francophilic, Catholicized Saigon elite: something that had deep consequences for the nature of his rule, especially when it came to his inability or unwillingness to tackle the corruption sucking away the fabric of the South Vietnamese state.

Now, your question: while unquestionably dependent on the US for survival by the mid-1960s, just as the North Vietnamese were hardly controlled by Moscow and Beijing as per Cold Warrior thought, South Vietnam was far from simply the extension of US will that anti-war activists alleged. Saigon could and often did act counter to American desires in a striking fashion, as did many client states during the Cold War. It just happened to be far less able to take aggressive action on its own than the North by the time the Americans intervened. Lot of reasons for this, but one of the biggest was the endless cycle of coups and internecine conflict between the leaders of the RVN following Diem's downfall. They took their toll: by the time Thieu managed to put a semi-end to the whole mess in late 1967, South Vietnam was literally on American life support, with vast swathes of the country de facto being governed by the insurgency. ARVN also never had, for a bunch of reasons-different geography, far more complex ethnic situations, systematic corruption, etc-the same coherent fighting ability that the ROKA developed, even before the Park Chung Hee years.

The thing to understand is that, unlike the different sides of the Cambodian Civil War to the west, the two Vietnamese sides basically had the same end goal in mind for Vietnam. Even highly anti-Communist Vietnamese wanted an independent, modernized, nationalist Vietnamese state that dominated the region and was visibly free from foreign meddling: which was part of what undermined the Saigon regime, propaganda-wise. The main disagreement between North and South was what model was to be taken to achieve this: the Soviet/Chinese or the Franco/American one. The former would win the war, but the latter would win in the long run. After the reunified Vietnam nearly became a failed state on the verge of mass famine in the 1980s thanks to Communist policies and the lingering aftermath of the war, a new generation of leaders adopted capitalist market reforms about a decade after Deng Xiaoping did it in China, with similar success down the road. Nowadays, Vietnam is in the same hot-house period of growth that China was in about a decade ago.

Sources:

Mentioned above.