It is so interesting that neither the novel nor the movie makes any effort to show Vito being sponsored by another Mafioso. He kind of just decides on his own that he is going to be a Don which is not consistent with the way the Mafia works.

As I've posted elsewhere and as some have posted here, after Fanucci's attempted intimidation and with the haunting memories of Sicily in the back of his mind, he decided to make he and his family as invulnerable as possible.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."