What if instead of shooting Robert Kennedy, Sirhan shot Reagan?
Possibly RFK defeat of Nixon in 1968. Since Robert was to some extent running as a 'Peace Candidate' that might mean him accepting the North Vietnamese peace offer in 1969. The deal that was signed in 1973 took four years to negotiate -and was functionally identical to the offer available in 1968. The differences - which supposedly prevented the North from conquoring and absorbing the South - gave Henry Kissenger something to excuse keeping the war going for four years, with a doubling of casualties in the process.
If the treaty had been signed in 1969 the US might have been more inclined to try to enforce it and to support the South Vietnamese Government instead of cutting military aid, removing the US contract personnel needed to maintain the ARVN's helicopters, aircraft etc as happened after the signing IOTL. In any case, Robert Kennedy was a very different man from Richard Nixon, and would have been facing a very different situation from Nixon IOTL.
Tens of thousand fewer KIA, military spending reduced somewhat due to not being still an active participant in the war, and a possibility of South Vietnam surviving as an independent state.
That is a very different world, and possibly one where the major US political parties might not have started their shift to where they are now. The Republicans still had a Liberal Internationalist wing after all, and the Democrats had a lot of right-wing supporters in the south. Those might have survived ITTL.