I think money, Vito knew the value of money for two reasons:
1. He grew up as a very poor man.
2. He quickly understood that money was a means to buy power, which is one of the motifs of "The Godfather", both film and book. Vito believes: If democracy is corrupt, I'd be a fool if I wouldn't play the game of the big shots.

In the novel he's German, but they refer to him as Irish.