Ah, sorry, but no -
- actually BEFORE the 1950s the concern was for Chinese Communist hegemony in IndoChina. Do you believe the concern for the "domino effect" started with Vietnam? I guess you also believe Truman nuke Japan only to hasten the war's end, and Korea was just a "one-off?"
Which all indicates that Vietnam had the motivation, possible the military capacity, for more but after a decade-plus of fighting perhaps not the will. Not just the will drained by the fatigue of war, but the will in face of pretty assured turnaround by the US should Vietnam move outside of Indochina (I think Nixon/Kissinger would have nuked them if they had entered Thailand - just like Truman, sending a broader message - and most hippies would have cheered).
"More" could have been the entire of SE Asia, including Singapore, and controlling the Malacca Strait where 1/2 of the world's oil is transported and 2/5s of global trade. From there, as a puppet of China, perhaps they would join to make sure the China Sea was fully theirs by taking Taiwan and the Philippines and threatening Japan and S. Korea, if not Australia. From there, why not take on India by making a lot of mischief in Burma, Sri Lanka and Bangladesh particularly in coordination with their allied Pakistan? and then a gateway to the Middle East; would there be any reason to doubt that Iran would be a fully nuclear armed nation today with ICBMs? A very very different and much more precarious world that would have had impacts even on retirees in Somewhere, Michigan. Today, instead, said retiree can take a really nice vacation on Da Nang beaches - I recommend it!
All of this is of course speculation and unknowable. That doesn't preclude the FACTS that Vietnam had the potential except for its decade-plus long drain of its war with the US. It should at least be a consideration in one's analysis of the consequences of intervention if one is truly interested in the question.
And that, of course, is the argument from ignorance. That's not meant as a put down; it's a fact that one cannot know what would have happened without the US intervention. One can speculate based on the facts of a military superiority and will of the Vietnamese with their Chinese and Russian backers. But, I certainly realize you can point to both Putin and China's lack of ambitions today as suggesting otherwise.