One JFKA CT narrative is that President JFK had decided to withdraw from South Vietnam, and so the military-industrial complex, along with the CIA, had JFK waxed, maybe through the cat's paw of angry anti-Castro exiles who felt betrayed after the Bay of Pigs.
The narrative has never really been compelling, as JFK had barely indicated tentative plans to withdraw from SV, and even if there were less boosts on the ground, US aid would still pour into in SV, in the form of naval and airpower, intel, diplomatic support, war materiel and money.
Another problem I see with the Vietnam-withdrawal-as-murder-motive narrative is that NSAM 263 was not released until October 11, 1963, just five weeks before the assassination. However, the plot was clearly already in motion by the end of September.
A more plausible scenario is that the plotters were
partly motivated by JFK's declining support for Diem, by the increased attacks on Diem coming from JFK's advisers starting in April 1963, and by JFK's acceptance of Averell Harriman's disastrous coalition-government deal for Laos in mid-1962.
If the plotters read NSAM 263's supporting documents, they knew that the withdrawal was expressly conditioned on progress in the war effort, that the withdrawal was gradual and would be paused if conditions on the ground worsened, and that the withdrawal called for an end-state of leaving 1,500 support troops in South Vietnam and of continuing military and economic aid to South Vietnam.
These facts are stated in black and white in the supporting documents, yet many leftist conspiracy theorists ignore them and insist on claiming that JFK intended to unilaterally and completely pull out of South Vietnam after the 1964 election and that he did not care if South Vietnam fell under communist rule as a result.