Once I actually took a novel and figured it all, considering novel only, no sequels and other bullshit. Now, what I got:
When Mike leaves Kay in the hotel, it is said that if someone told her that she will not see him until 3 years passed...
So, he spent about 2,5 years in Sicily and 6 months before she contacted him.
Then we find about his trip to Vegas: "It was almost 3 years now since he had returned home and over two years since he had married Kay." 5,5 years since the murder.
When he is back from Vegas he tells the Don, who asks him "when will you start?" - "A year from now." And then we read: "He was patient, to use the full year to prepare. But he was not to get his necessary year, because..." Here the Don dies. And after the funeral Mike says to Tom: "I needed about four more months..." So that was 8 months more. 6 years and 2 months. If the murder is near the New Year eve 1946 , that makes early spring of 1952 the date of the Great Massacre. So, Vito died in 1952.