Vito was the man in mafia terms. His family would have NEVER been touched had he never went away to prison in America and that is GUARANTEED! That tells you enough about his power, that the power grab was put into motion when he was away. That was the only way they could get anyone to go along with the plot to begin with.
What tells me about his power is that when he came out of prison he pretty much killed all his enemies within a year.