Ok, this is more or less my musing, less debate backed up by any credible research papers (news, historical shows, documentaries, articles, journals...). it may or may not recycle the ideas already posted in this thread.
As mentioned on page 3 (or 4), someone defines the "win" condition for the US is to have an existing "Republic of Vietnam" with a permanent division, making the two countries de facto independent states. Kind of like Taiwan and PRC, or DPRK and RoK at the moment.
The RVN, officially speaking, starts its existence in 1956 (on a direct violation against the Geneva Accord of 1954, but the State of Viet Nam did not sign the Accord, the violation is legally accepted) with a referendum. The one that Diem wins against then-king Bao Dai with a 600k votes in favor of him (Of course, let's butterfly and ignore that the fact that there are only 450k eligible voters in RVN then, "magic of democracy"). For ease of reference, I will take 1954 as the benchmark, or to be specific, Jan 1954, when the preparation of Viet Minh (effectively the armed forces of DRVN then) is being re-done for Battle of Dien Bien Phu.
Personally, short of having some military actions (as in boots on the ground) against DRVN, it will be hard for a surviving RVN. Note, it can also be the deployment of special forces and saboteurs (which happens IRL). However, in the context I'm talking, it should be deployment of US Airborne (or equivalence) in Battle of Dien Bien Phu. Having a less catastrophic failure in DBP (and later, in Man Yang Pass) will give the France (and their financier, the US) more leeway on the negotiation table. Perhaps a "fuck no" to referendum. And while they are at it, make a thinly veiled threat for a deployment of nuke (it was nothing more than small talk IRL).
Assuming that the Geneva Accord still goes on as RL (maybe the DMZ is on 15th parallel, the US and the State of VN still do not sign the Accord), future-RVN would be in a much better position. However, things are still hung in balance for them.
First off, Diem MUST NOT show any discrimination against any religions, accidentally or not. The crisis of 1963 must be avoided, because it is what push multiple Buddhism monks into the supportive stance of NLF (more commonly known as VC in the West). In addition, to increase the success chance, Diem should launch false-flags operations, using (fake) Buddhism monks to antagonise areas that already supportive of NLF, this would definitely deteriorate the relationship between the two (though it should be done with care and ease, Buddhism has been in Viet Nam for nearly 1000 years, so by existence, the NLF has more in common with Buddhism, compared to a Catholic gov of RVN.
Then, political stability. This also includes the US ceasing some (or preferably, all) of their command of their troops to RVN. Having direct combat forces from foreign countries is a major selling point of NLF to point out that the RVN is a puppet regime. If Diem can put all US forces under his command (which consists of just US "advisors" in 1954~1960), it would help, a bit. There are also other factors, such as economic equality, the control of economy and finance carried out by Chinese ethnicity, or (again), religion discrimination.
Third, under no circumstances, accepts the bombing of DRVN. As long as DRVN is bombed by USAF, the game will be lost to the US. Bombing the country will not pushed the civilians to rise up and riots, they will simply make them more determined to fight back, even if the reason is vengeance. In order to have this, the Gulf of Tonkin Incident must NOT happen. Good luck with that with the Red Scare in the US.
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Personally, though, the US can "win" by having FDR living for a few more years and NOT having Truman in power. This is 1945 we are talking about.