I think people underestimate the degree to which the Tet Offensive actually alienated South Vietnamese opinion against the PRG and the North. The devout Buddhist population, for what it is worth, regarded the actions in Hue to be sacrilegious, and they had previously been vehemently against Diem and his successors. There WAS a shift in public opinion on the ground in the South, and there is a reason why the NVA had to be infiltrated to take up the positions formerly held by the insurgents, who to a large degree had been decimated by Tet. In fact, debates about whether Tet was supposed to reduce the power of the PRG and VietCong in proportion to the leadership in Hanoi, who often saw things differently both tactically and ideologically from the southern insurgents, has gotten a lot more attention after records decommissions and reveals in recent years.
The idea that there was no way that the South could have won is in my view, inaccurate, after Tet. Before Tet, you had a broadly unpopular government fighting a movement that was more popular and better attuned to the regions it operated in. After Tet, those patterns were completely disrupted, a large proportion of the population became internal refugees dependent on the (admittedly extremely corrupt) government in the South, and the power of the Viet Cong over areas in the Mekong Delta and mountainous west central regions of the country diminished. The security status of the South after 1968 (and further, high casualty, low yield on the ground, offensives) was actually not that bad, and it was good enough that the war became a regularized stalemate with the South Vietnamese government broadly more powerful and influential over events in the rural parts of the country, with much of the conflict switched into neighboring countries over the flow of NVA regulars and NVA equipment southwards.
The South Vietnamese were not vulnerable to guerilla action overthrowing the government after Tet. What they were vulnerable to is large scale conventional offensives using massed armored and artillery support. Much of their population were internal refugees who clogged the roads and the ability of ARVN to respond to large scale offensives. Once it became clear that US help was not coming in the form of aerial bombardment with B52s to smash the formations of attack, there was a panic in much of the country and this only exacerabted the problem.
The South Vietnamese could have survived for as long as the USAF was there to smash large scale offensives with intense napalm and carpet bombing efforts. Localized offensives could have worked for the NVA, but they would not have been able to tip off the kinds of panic that they did in January 1975. Keep in mind that Hanoi was surprised by the collapse in Da Nang and the Central Highlands almost as much as anyone else was. The South had numerical superiority for almost the entire time up until the fall of Saigon. But it evaporated pretty quickly once panic set in.
The decision to focus on the "core national areas" by Thieu after the initial border fighting was a disaster, as it created situations akin to the Six Day War pullback to Suez happen where units lost complete cohesion as they streamed to what they were told was safety, but only exposed them to flank attacks already underway.
So really, this should not be too hard. You need American bombers to show up, and it would help if there is better ARVN leadership.