He is indeed a composite of various mobsters, but in particulary Frank Costello, Carlo Gambino, Lucky Luciano and Joe Profaci.

His personality was said by Puzo to be based on his mother. But I'm sceptic as to how serious he was about that statement.

But his behaviour/beliefs are cleary inspired by old time Sicilian mob bosses from the early 20th century.

You can find more about this here:

http://godfather.wikia.com/wiki/Vito_Corleone