Fair enough. However, there was no need to refute the testimony, because the "testimony" was not evidence: It was not taken under oath. When offered the opportunity [with immunity] to support their claims, the VVAW refused to do so. Since the investigation of atrocities often involves a crime scene terrifically confused by age, tampering, locality, and fabrication, physical evidence is often secondary to the accounts of eye-witnesses. The fact that none of the VVAW allowed themselves to be deponed under oath is, in itself, interesting.
I, for one, don't really think it matters if Vietnam soldiers WERE committing war crimes, because I don't think the Geneva Conventions should take precedence over US sovereignty. I admit I don't know too much about the details here -- whether free fire zones really are "illegal" under international law, etc. -- but I don't care, either, except out of pure curiosity.