By "got to him," Sol meant, "killed him." Before he said, "10 years ago..." he said, "Let's face it, Tom, the Don was slippin'. That tells me that 10 years earlier, Vito, having recently fought a life and death gang war (novel) would have been a lot more careful about being bodyguarded, and about being careful of whom he dealt with.

IMO, the real slip Vito made was sending Luca to Tattaglia with that ridiculous story that he was "not too happy" with the Corleone family.