In "Slow Burn: The Rise and Bitter Fall of American Intelligence in Vietnam", Orrin DeForest cited speculation from South Vietnamese sources that Vann's death was caused by disgruntled South Vietnamese Rangers. "They hated his guts up there", he wrote, and added, "Vann was an abrasive man with a foul mouth, given to embarrassing Vietnamese commanders in public. He'd meet a commander of a unit out in the field some place and he didn't care who was standing around. He'd tell them right there, 'Hey, you dumb son of a bitch, you couldn't run an operation against the VC if you were the last one in the world.' It made no difference to Vann that 'face' was the most significant thing of all to an Asian. You simply cannot embarrass a Vietnamese, at least not in front of someone else. Not unless you are ready to make an enemy for life, one who will get his revenge if he possibly can. He will watch and wait and be patient, and he'll just brood on it until he finds a way to do it."