Not that disturbing IMO. Not really as edgy as it was considered to be back when I was in high school and all the goth and counter culture kids were all about it. There is one scene depicting a rape and some gang violence near the beginning.
I believe that the rating would have come about more for the fact that it was highly antiestablishment, yes the violence and rape scene were extreme for the time it was filmed. But I think it was more the torture to 'make him a productive member of society" that would have had the raters hating it.
Never heard of Vendetta. I saw the car logo and was typing every movie I could think of that had anything to do with cars - Ford Fairlane, American Graffiti, Talladega Nights, Christine, Days of Thunder, Fast and Furious, then thought to self, "What other movie has something to do with cars?" Sometimes the simplest answer is the best answer.