Yeah, they did. Of course, the ones who did it openly paid dearly for it, and there were a few incorrigibles who the N. Vietnamese just couldn't supress (Bob Schumaker was one, I think) but they beat them to within inches of their lives.
Then, of course, there were things like the guy flipping the bird with both fingers while his picture was being taken. And the guys who did it openly early on, telling their captors it was the "good luck sign".
I don't think they did it like the muslims in Gitmo apparently do to their guards. There are things the guards can do, but hanging them from the ceiling by ropes tied to their wrists behind their backs isn't one of them.
The Vietnamese used the strappado torture method on Americans? That's terrifying stuff.