While the dog could also be "nasty", here the meaning is that the dog is "mean". The two words are different. You could try reporting it. “Nasty” usually has a connotation of “disgusting”, but some people also use it in place of “mean.”
Include this dictionary definition which will help your case in your report:
Question for native speakers: is it typical to use the french adjective vicieux to describe a vicious dog? To my English ears méchant can mean badly behaved in a more general sense, not just viciousness.
In other words in French is 'méchant' generally understood to mean vicious in relation to dogs, or would vicieux be more accurate?
Thanks for that, but I'm not sure I made my question clear enough. I agree that 'méchant' means more than just badly behaved; the most accurate translation of that word is, I believe, 'wicked' although obviously it depends on the context. So the most appropriate translation of the above sentence might be 'wicked' dog, not vicious.
My main question really is, when a french person wants to warn you about a dog that might bite or chase you, do they use 'méchant' or 'vicieux', or “un chien qui se comporte mal”?
What is the most commonly used idiomatic french phrase here?