Vito speaks to this issue himself in the first scene of the movie. After Bonasera talks about how he came to America and raised his daughter in "the American way," Vito says "lets be honest here" and goes on to say that Bonasera made a good living, and he thought he had the police and the courts to protect him and so he didn't need "a man like me."

Vito traded in corruption. His strength was his political power.
Note that all the judges and congressmen who were invited to the wedding did not show up but they did send gifts. When Vito granted the baker's wish, he casually tells Tom to "give this to a jew congressman not in our district.

For Vito then, the concept of "country" was non existent. I am not sure about what is meant by the term "unamerican," but Vito was born in Italy and he never bought into the American system.