I agree with you, the “Don was Slippin”. It certainly wasn’t a very well thought out plan of Vito’s to think that anyone would believe that Luca had turned against the Corleone’s . Vito clearly knew ahead of time that his decision to turn down Sollozzo’s proposed business was going to mean trouble, because why else would he have Luca already at the office the day of the Sollozzo meeting? At the least he should have been more cautious in the future about a possible hit.

Something else that I’ve wondered is why did Vito include all his top people in the meeting with Sollozzo, but go it alone with his plan afterwards? Was Vito worried, that his top people might turn against him over the drug business? It’s possible that he eventually told the others about sending Luca on his mission, but we don’t know if he absolutely did. The meeting after Vito’s attempted hit showed that they didn’t know where Luca was or whether he was still loyal.