Counterinsurgency broke the news after Petraeus and McChrystal, but it was actually very big in the popular and military imagination during the '60s as well. Mao's success in China sparked a huge discussion of how to conduct and defeat insurgency. For obvious reasons, the Vietnam War continued this trend.
Reading A Better War, a classic revisionist account of Vietnam, is like looking at Iraq all over again. The initial commanders in both wars--Westmoreland in Vietnam, Tommy Franks and others in Iraq--were basically clueless about how to tackle local insurgency. Westmoreland, for instance, focused on body counts and firepower. He threw artillery and air power at guerrillas, and focusing on winning battles. He exuded optimism and threw out made-up statistics during press conferences. As in Iraq, this led nowhere.
Then you had new generals in both wars. In Vietnam, General Abrams came in. He replaced "search-and-destroy" missions with ones focused on "clear-and-hold" to implement a policy of civilian defense. He focused more on training native Vietnamese units. This coincided with Nixon's "surge" strategy of escalating the conflict to leave on better terms. The dramatic change in policy as a result of following counterinsurgency doctrine and the dramatic change in the outcome of the war in America's are not in question. Needless to say, the surge under Petraeus and Odierno, and the shift in American doctrine towards COIN, was a similar story.
The disputed part about Vietnam Revisionism is whether Abrams' efforts were enough to win the war in the long run. North Vietnam enjoyed enormous support from Communist states, while Congress eventually killed all military aid to South Vietnam. In the Revisionist counterfactual, limited military aid would have allowed an insurgency-free South Vietnam to sustain direct attacks from North Vietnam.
I think a fairly convincing argument can be made for that scenario. But in any event; there are better and worse ways of fighting insurgencies, and it takes a while to develop the competency and leadership capable of fighting them.
The success of counterinsurgency in Vietnam and Iraq doesn't imply that it will work in Afghanistan, a very different country. Nor will it tell you if Vietnam, Iraq, or Afghanistan were ultimately worth fighting. Just because a given tactic can win a war doesn't mean the war is worthwhile.
But from the point of view of South Vietnam, at any rate, it probably was. South Vietnam remained mired in poverty for twenty years after its war, while it enjoyed growth under a free market before the war, and would probably have enjoyed even greater success had it won the war--something like a North Korea/South Korea analogue (I might even go so far as to say that if you think the Korean war was worth fighting, than you should have supported South Vietnam in 1975, even if you opposed sending troops in during the 60s).