I'd say the Don wasn't even as personally violent as Clemenza either, but he did have the courage to do what Clemenza and Tessio wouldn't and that was kill Fanucci. I believe that the novel states that after Vito kills Fanucci that Tessio and Clemenza avoided him for several weeks and then came to him in obvious respect. I think at that point the two of them realized that Vito had the foresight and cunning that they might have lacked. I don't think Vito gave the two of them any evidence of his future skills before murdering Fanucci.

Luca is obviously a much more complex character than both Clemenza and Tessio. To be honest, I don't have an explanation for Vito's handling of him at this time because I haven't read the book in awhile and can't even find my copy. I am listening to the audiobook presently so perhaps I can provide some insight in a few days.