How long did it last? If the vietcong mostly always suffered 3-4x more losses and I use the word mostly, how did they sustain their army, fight over the course of a long war? So Saigon was taken the moment the Allies left, how on earth could anyone say that's a victory? Good documentary to watch on the Sniper battles?
As just a minor note, US combat troops were withdrawn from Vietnam in 1972, although air involvement continued for some time thereafter. The invasion of the south was commenced by the North in March 1975. Any of the texts that cover the entire war will cover that.
I note that, as it isn't the case that Saigon was taken the moment the "Allies" left. Foreign combat troops withdrew in 72 and the Saigon fell in April of 1975, so there was a more or less three year gap.
I'm not saying that was a victory (for the west), merely noting that there was an interval in there. It might be further noted that the fall of the Republic of Vietnam came by way of a conventional invasion of the South, accompanied by the decision of the United States not to intervene with air assets, in contrast to the 1972 NVA offensive in which US air assets were deployed.
Coincidentally PBS is currently running Ken Burn's documentary on the Vietnam War which might be a good one to catch regarding some of these questions, such as when the war can be said to have started, which is somewhat difficult to really say. Large scale US involvement commenced in 1965, but the war was going on prior to that and to some degree you can perhaps go back to the 1954 Geneva Accords which partitioned Vietnam and which were disregarded in varying degrees by both the north and the south relatively quickly. A guerilla phase of that started soon after the Accords and escalated over the years thereafter.