I define vicious as unprovoked attacks.
Guess I’ve been around too many mastiffs in day. The and the coonhounds.
I think that we would have a different definition. A lap dog could nip at you in an unprovoked attack. Annoying but not threatening, so in my book, not a vicious attack. I see a vicious attack as having a more serious outcome. I see vicious as a serious, determined attack that is not easily broken off, even by the dog's handler.
Unprovoked is difficult to judge. The person may have no idea why the dog attacked but to the dog, there was a triggering event that was clear to it. Remember yesterday on FR, there was a pit bull attack that was allegedly triggered because the guy's girlfriend had her teeth whitened and she smiled at the dog who took the smile as a threatening growl and attacked. She had no idea this would trigger the dog who savagely mauled her and torn off her nose.
English mastiffs, I assume. Much more domesticated, if you will, than most of the other mastiff breeds whose aggressiveness has not been bred out.