I'm gonna piss everyone off with this reply
Assuming Tricky Dicky stayed in power through both terms, there might not have been quite as much of an American stampede to get out of Vietnam.
I argue that without Nixon politically on the ropes, there would have been more obvious American support of RVN and Vietnamization could have been more successful.
No question that RVN had major social issues preventing it coalescing into a united nation with a purpose beyond denying the Viet Minh their political victory from 1954 on.
The long and short of it is, I argue that with America not quite so eager to hit the trail, and maintaining a presence in South Vietnam, the NVA wouldn't have been so froggy to commit their forces in 1975. By 1978, ten years have passed since Tet so its impact will have faded.
IMO, with substantial restructuring of the ARVN into a more professional mobile, well-armed force with organic artillery, CAS, and tanks, splitting off a big chunk to serve as a gendarmerie/Civil Guard and part-time militia--
chances of a Korean-style stalemate go up to the point that South Vietnam could have gotten enough room to economically and politically establish itself.