Sollozzo waited four months after Sonny’s gaffe at the drugs meeting to make his attempt on Vito. Why did he wait that long? The answer, IMO, was that while Sonny was the ch*nk in the Don’s armor, Luca was the armor. Sollozzo had to find a way to eliminate Luca before he tried to negotiate with Sonny. Luca was more important than Sonny in Sollozzo’s plan. And that scenario shows us where the Don was really slippin’:

When Vito turned him down, he presented Sollozzo with an inimical, “I win/you lose” situation. Sollozzo immediately figured he had to kill Vito. Sonny’s indiscretion gave him hope that he might persuade the Corleones to go along with him. But he knew he couldn’t survive to negotiate with Sonny unless he first eliminated Luca. He knew that nothing would stop Luca from seeking revenge. Hagen told him something he already knew when he said, “Not even Sonny’ll be able to stop Luca.” Luca might even have come after Sonny if he’d tried to negotiate a deal with Sollozzo after Vito’s death. So, he needed all that time to find a way to get rid of Luca first.

And Vito handed Luca to him on a silver platter:

If Vito had told Luca, “I’m a little worried about this Sollozzo fella—kill him,” Sollozzo soon would have been dead. But by telling Luca to “Go to the Tattaglias…make them think that you’re not too happy with our family…” Vito was arming Luca with a ridiculous story that no one would believe. Every Mob guy in New York knew that Luca worshipped Vito, and would never betray him. They also knew that Vito would never let Luca—his strong right arm, his “most valued friend” [movie], “one of the great stones in his power structure” and “a man who could do a job of murder all by himself” [novel]—proffer his unique services to a rival family.

So, when Luca made his first overture to Sollozzo and Tattaglia, they knew immediately that it was bull***t. It showed them that Vito was running scared, and not thinking strategically. What’s more, it gave them plenty of time to set a trap for Luca. He showed up at Tattaglia’s club that night seeking to play out a deception that never deceived them. They showed up fully prepared to kill him—and they did.

With Luca out of the way, Sollozzo was free to attack Vito without fear of retaliation from the Corleones’ most implacable defender. And, formidable though Sonny was, he was thrust in the position of acting head of the family—a position he was ill-equipped to handle and that left him unable to command his troops on the street. By eliminating Luca, Sollozzo gained the psychological upper hand, and cleared the path to making Sonny and Hagen consider his proposition by showing them that their armor had been stripped away.

So, when he told Hagen that “The Don, rest in peace, was slippin’—ten years ago, could I have gotten to him?” Sollozzo didn’t just mean that Vito had screwed up his bodyguard detail on the day he was shot. He meant that he’d discovered that Vito had made himself vulnerable by relying too heavily on Luca for protection and Sonny for succession. Too bad for Sollozzo that he slipped up by underestimating Michael.