I believe that there are. I also believe that, given our country's attitude toward the soldiers who did return, that there may even be a few veterans of that war who saw how their fellow soldiers were treated upon their return and decided to forget about the US entirely and make new lives elsewhere. And I for one would not blame them a bit.
There is no valid prove as to whether there are still prisoners of war in Vietnam currently. However, POW doesn't necessarily mean that they live in foreign countries. The pain, horror, distress and nightmares make them forever Prisoners of War even when they get back to their countries.
Absolutely, whether they're still alive is anyones guess. For their sake I certainly hope not, but if proof turned up that a living POW was found, damn straight I'd volunteer in any way, shape, or form to help out