It's simple, as long as Vito lived, they couldn't seek revenge due to his word he gave to the other Dons that he would not start a new vendetta. As a man of principles, Vito couldn't break his word.

And I just stick with Michael returning to the states in '48, since it's said multiple times in the novel that he was gone for two years. It's said at least three times, more than the 6 months back which is said only ones by Mama Corleone. And in the movie, Michael was back for a year when he reunites with Kay.

But another thing that bothers me is that Kay says in Part II that Michael promised her that the Corleones would be legitimate in 5 years, which was then already 7 years ago. This means Michael reunited with Kay in '51, not '50 since it was 1958 when she said that. And since it was clearly autumn when they reunited, it must have been late 1951, which means Anthony must have been born in 1952, not 1951 as the DVD and Blu-Ray says. But then he must have been only 6 in 1958 when he got his first communion. Won't a child get his first communion at the age of 7?

This is getting too complicated... Fuck it.. Kay and Carmella just made mistakes, that's it, lol.