I was doing pvp against a buccaneer. My el toro (masked champion, 65) attacked the buccaneer, and was hit by vengeance strike after. He only hit her once. She did her vengeance strike and i saw he was stunned (stars around his feet, another glitch it should be his head lol) and she somehow hit again, with the same "Vengeance Strike" appearing above her head. So basically he hit her once, and she activated vengeance strike twice. I think it was because she stunned him, since during other pvp matches it has happened only when they were stunned by vengeance strike.
I've see enemies hit with two or three Vengeance Strikes on the same character and some of my Companions have done the same. I think it depends on the level of Vengeance Strike that the character has. If he has a Vengeance Strike of 2, he could hit for two times. If he has a 3rd level Vengeance Strike, he could hit the same character 3 times.