While you're waiting for other answers, you might enjoy these posts by /u/LordHussyPants, as well as the ensuing discussion:

• Were songs that we currently associate with the Vietnam War (eg. Fortunate Son, All Along the Watchtower) actually popular during the War, or has the association just come from movies about it?

• In many Vietnam war films, Credence Clearwater Revivals Fortunate Son can often be heard, but was the song popular among the soldiers? Was music like that allowed to be played on helicopters?

(side note to /u/LordHussyPants - I was on my old account when I called that answer excellent, and I still think it is!)

I was curious about this as well, and after looking into their discography, I think part of it has to do with the sheer volume of music CCR put out in that time. All 7 of their studio albums were released between 1968-1972 - 9 of those songs were top 10 hits with another 7 in the top 50.

That being said, for as closely associated as one would think their music was to the war, very few of their songs actually appear in media associated with the war. After reviewing IMDB, there's only a handful of movies/shows/documentaries that CCR appears in about Vietnam or in Vietnam scenes. Starting in 1987 Dear America: Letters Home from Vietnam, Air America (1990), Forrest Gump (1994), and Tropic Thunder (2008). However, none of their songs appear in any of the most popular Vietnam movies such as Platoon, Apocalypse Now, Full Metal Jacket, Deer Hunter, Good Morning Vietnam, Hamburger Hill, Born on the Fourth of July, Casualties of War, Rambo, etc.

I truly believe it was the popularity of the movie Forrest Gump and the scene transitioning to Vietnam where Forrest is flying in a Huey while Fortunate Son is playing that has left this lasting impression.