http://nypost.com/2014/09/21/how-star-wars-was-secretly-george-lucas-protest-of-vietnam/
Also discussed a bit in the documentary/book Easy Riders, Raging Bulls, and I think they might even have talked about it a bit in People v George Lucas. And apparently in the book referenced in the article.
Well, I know George Lucas commented on it in that article (heck, that was where I got irrefutable proof that Lucas had based not just the Ewoks, but also the Rebels on the Vietcong ever since Episode IV was in development, back when it was simply known as Star Wars), though I don’t recall Coppola mentioning the Vietcong ties in there, which was what I’m asking about.
I will look into Easy Riders, Raging Bulls, though.