I'm bumping this old thread to add some recent info I found along with some photos of the "Rock Apes".
First, here are a couple of photos of what the soldiers serving in Vietnam called a "Rock Ape".
And here is a question and answer from the fellow in the photo holding the Rock Ape.
So it seems some of the mystery of the Vietnamese Rock Ape has been solved. They apparently were some sort of unidentified monkey erroneously referred to as an ape.
I say "some of the mystery" because there are other eyewitness reports that don't match the monkey pictured in the photo. And it seems perhaps the one in the photo is an unidentified species, at least from what I can find.