To those who are comparing South Vietnam to South Korea and claiming the former could have been put into a similar situation, i'd like to dispute the issue through maps:
Korea is peninsula. The only land border they had with another country, North Korea, was and is still a heavily fortified demilitarized zone. Any guerrilla fighters that would want to bypass the DMZ to infiltrate the country would have to do so by boat, which is a tall order considering you'd be exposing yourself and your party to the open skyline, air and naval patrols. The country is also not that underdeveloped, having inherited many Japanese industries that the government seized, done land reform successfully through the 1950's, diversifying its economy, and competently handling a steady stream of western investments. Geographically equal urbanization, with numerous urban centers (Taegu, Taejeon, Busan) peacefully developing alongside Seoul also helped.
Compare this with Indochina:
This is a huge place. To the west of South Vietnam is some quite ideal guerrilla territory in the form of jungle forests, mountains, riverside communities, rice fields, etc, into which rebel bands can easily escape to and hide in when an attack goes sour. This was the rationale behind napalm usage; to destroy the foliage, potentially burn any VCs hiding in it, and leave the area an open, scarred field, risky for a soldier to walk across. At the same time, though, it came with its own downsides in the form of collateral damage to farmers' crops, fishing and hunting grounds, and massive ecological damage, which hampered efforts to gain the goodwill of the rural population. What could be passed for a large city was Saigon, and it was surrounded by a large countryside, with war sometimes flaring just a few kilometers from the city suburbs.
If Vietnam was a peninsula and Cambodia and Laos were instead underwater, perhaps the US and SV could have beaten back the northerners and the VC, but overall, i feel like geography is an overlooked factor in influencing the outcome of the war. The intervening forces were faced with the dilemma of either keeping counter-insurgent operations limited to South Vietnam proper or expanding the war into Laos and Cambodia to pursue insurgent armies and infrastructure which would destabilize the region and require even further reinforcements and escalation.
Here's a little article from 1970 that illustrates my point more eloquently.