Is that to say that the US isn't going to try another Tet-style offensive? I'd think that, due to the fighting insurgencies, which we weren't to good at, we'd try some sort of massive blitz toward Hanoi...which would probably collapse, but would do enough damage that the Vietnam War would become the exact same as the Korean War before it.
Think of the long term effects though. Supposing the best is a divided Vietnam, communism in the rest of Indochina changes. Laos may still become communist, but a more confident US will never let the Khmer Rouge come to power in Cambodia, especially not as a neighbor to their ally in South Vietnam. No Khmer Rouge means no "killing fields" and several million people aren't executed. All and all, perhaps a better 20th century in Asia...
Well, considering it was the North Vietnamese who launched the Tet Offence I'm not sure it's entirely up to the US.
The best possible outcome for Viet Nam, with a POD starting in 1968, is for the Tet to happen as it did historically
but have it viewed as a success in the USA. The USA can than follow the Nixon/RFK Vietnamization plan and have it work as it did IOTL (until Watergate cut off support to the South Vietnamese). Tet being viewed favourably also means we avoid Nixon's concurrent extend the war plan which didn't help much.
As mentioned this avoids Pol Pot, the boat people, and the various effects in SEAsia that came from the collapse of South Vietnam.
The next best possible outcome is simply to have RFK live, win the nomination, win the general, and implement Vietnamization faster and earlier than Nixon did. RFK isn't going to have Watergate, and so (like Taiwan & South Korea) the South Vietnamese will have the money, weapons, and quiet (air power, special ops) military assistance they need to stay stable and beat the North Vietnamese if they invade.
However this timeline supposes no Tet, which leaves the US forces in the unhappy position of failing to beat the Viet Cong without stepping it up (and thus, probably, kicking off the PR backlash that Tet caused IOTL) and having the North Vietnamese win by proxy.
If there is an analogue to Tet sometime soon and it also causes the destruction of the Viet Cong and it does not trigger the Viet Nam backlash in the US that the OTL Tet did… than sure, things could work out well.
However the most likely outcome from no Tet is a Viet Nam war where the US cannot pull out (Viet Cong), cannot win, and it gets more and more brutal.