The discussion over on the movie side brings to mind two things about Luca that are more developed in the book. The first is that Vito is not entirely comfortable with Luca. The second is that Vito's insight is that Luca is essentially suicidal. When he sends Luca to spy on the Tataglias, Vito has probably been resigned for many years that some day Luca would not come back, and Vito probably doesn't see that as such a bad thing.