The argument that the US won the war in Indochina rest on the assessment that the US had accomplished it's strategic goals by 1973 and that South Vietnam and, to a lesser extent, Cambodia were capable of defending themselves against the Soviet and Chinese attacks if provided logistics and air support.
We were never going to win the war in Vietnam like we won WWII because the peoples of South Vietnam and Cambodia were under constant assault by the Soviet Union and China. There was no way we could have brought a definitive end to the conflict until the fall of Communism in the 1990s. South Vietnam would have looked like South Korea.
In Vietnam, the indigenous Viet Cong were completely destroyed in the Tet Offensive of spring of 1968. Deluding themselves into believing that the people of South Vietnam would rise up and join them if they could only seize control even temporally, the North Vietnamese and their Soviet masters, threw the whole of the Viet Cong into a single roll of the dice to seize as much of the country as possible. However, the people of South Vietnam did not rise up, not even in places held for weeks. And the Viet Cong were virtually exterminated. The the mass murders, tortures and general brutalities committed by the Viet Cong while they did have control ended forever any remaining positive beliefs the people of the South had of the communist.
For the next 7 years, the war would be exclusively one of fighting off successive waves of invasions from the North.
With the change in US administrations came a radical change in US tactics. Westmorelands attempts to refight WWII in Europe with just non-Vietnamese troops were replaced by the Marines proven success with hearts and minds and Vietnamization. A vast militia system was set up wherein every small town and village was trained and armed to protect itself form North Vietnamese raids and terrorism.
In 1973, the US began a rapid draw down of ground troops who were replaced by the South Vietnamese. In early 1974, the North Vietnamese attacked across the border. The attack was bloodily repulsed by South Vietnamese ground troops supported by US airpower. South Vietnam was largely politically stable and had a functioning military. It could defend itself if the US provided an offset for the huge amounts of money and material the Soviets and Chinese were pouring into the North.
In late IIRC 1974, early 1975, Edward Kennedy in the US Senate and Tom Harkin in the House of representatives sponsored a bill that, despite our promises otherwise, cut off all military and humanity aid to South Vietnam and Cambodia. North Vietnam invaded the south with a conventional heavy military force using more tanks than Hitler used to conquer France. The south Vietnamese military held out until their ammunition ran out and it became clear they had been betrayed and left to face alone North Vietnam, operating as a fully supported proxy of both the Soviet Union and Mao in China. At that point, South Vietnam disintegrated as each family tried to save who they could. Very much like those who fled the German invasion of Western Europe in WWII.
The results of our abandoment of the defense of the people of Indochina are beyond the scope of the question and to horrible and extensive to detail in the space available.
Suffice to say it was every bit the equivelent of abandon Europe to the Nazi's and the Holocaust. Cambodia suffered the worse proportional mass murder in the 20th century with 1.5 million out of a population fo 5-7 million killed within five years. Everyone with education was killed and the country was left littered with mines. The Stalinist Ho Chi min murdered 180,000 outright, "reeducated" and starved unknown thousands more and drove millions of more out as destitute and exploited refugees. Endless atrocity filled intricate Communist war dragged on 20 years until the fall of Communism.
Our abandonment of Indochina was the worst thing that America has done since at least since the Civil War and probably ever. It was ever bit as cruel and callous as if we had known fully about the Holocaust of WWII and then just left the Nazi in charge of Europe.
Worse, we did so purely for matters of internal domestic social and political conflict.
Those who claim we won the war really mean that we had reached the point where Vietnam could defend itself with enough American help to offset the Stalinist. superpowers. This is completely true (as far as any version of history is true.)
The arguments that the war was impossible to win were just attempts to explain the strategic blunders and systemic mismanagement of two Democratic administrations. What better excuse is there for bungling a war than the assertion that nobody could possible win it? The end of the Cold War revealed that all the arguments against continue to support the people of Indochina were at best wrong and at times, simply delusional.
But the reality is that even though we could have won to the extent we won Korea at the very least, in actuality we lost and lost bad. We lost for the reason most polities in history lost wars, we focused on our internal conflicts and ignored the external enemy. We weren't divided by the war, we lost the war because were we were divided.
But we did lose, and there no sense sugar coating it.