Yea I agree with Olivant and I think that to believe that Vito did this because Luca would cause problems for his sons is overthinking the room. If Vito expected a hit on his life it seems that,though he was slipping as far as being on top of everything,he still would have been wise enough to knockoff the other dons before allowing his life or his sons lives to be put in danger. If he really thought that a hit was immediately imminent Luca would've been sent to whack out the Tattaglia's more so than find out what they were up to. The slipping would have been not being able to see the Barzini's behind the whole thing