As revealed in Vito's "Is this necessary?" response to receiving Luca at the wedding, Vito's actually ambivalent about Luca's role so close to him. We simply don't know how many times Vito has sent Luca into risky situations that Luca emerged from intact. In the book, Vito says something like, "Luca was such an extraordinary man that no one could kill him for a long time." You couldn't know that unless you had tested it out, and my guess is that Vito had long seen Luca as an expendable asset.