What if the US does not back out after Tet? Instead they successfully explain to the public that they were on the verge of victory. After a few months of hard pushes by the Americans and South Vietnamese the North Vietnamese are forced to surrender. What happens next?
There's a huge difference between preserving South Vietnam and making the North surrender.
The former is doable, especially if Watergate doesn't happen. Just have the American Air Force strafe any tanks going South of the border if the North tries to pull a 1975; armour and infantry are easy to target compared to the guerilla forces in the jungle. Maybe they'll try a few more times, but the principle would be established. South Vietnam will then probably become a democracy somewhere in the late 80s; god knows what would happen to the North, as they could easily swing reactionary like the Koreans in response to the Southern threat.
The latter is extremely hard, and borderline ASB without nuclear intervention. China won't exactly be up for the Americans on their doorstep, and might make a full intervention, not to mention the Soviets. The guerilla forces the United States would have to defeat would have skyrocketed in number, and we all know the problems they faced in South Vietnam alone. It would require hundreds of thousands of casualties, and I don't think the American public would be okay with that.
America's best shot in the war is immediate, Nixonian-style escalation to bring the North to the table, and then rigidly enforce the equivalent of the 1972 deal. Granted, it's still as morally murky as ever, but it's good for America's overall pride and self-esteem as a country.