Viggo Mortensen is relating a tale that involves Vince Vaughn, a butcher knife and the threat of bodily harm. But, oddly enough, the actor isn’t describing the shooting of his latest film, “Psycho.” He’s remembering a country music concert. “Vince and I went to see Buck Owens one night after we had finished that day’s shoot [for “Psycho”],” Mortensen said during a call from his Los Angeles home. “I had gotten one of the `Psycho’ knives to give to Buck as a present ’cause it was his birthday. They wanted us to give it to him on stage that night. “So we bumbled our way through our speech to Buck. He opened the box and saw the knife, and the fiddler started making the “eek eek eek” [noise from the shower scene]. Buck got all excited and started posing as a damsel in distress. Then he began chasing Dwight Yoakam all around the stage with the knife. I don’t think he realized it was real.”