When Michael moves Vito in the hospital, you can see tears streaming down Vito's face when Michael reassures him that he's being protected.

That probably can be interpreted a couple of ways - Vito was grateful that his family was watching over him in his time of need, or that he was sad that his hopes for Michael living an American life were gone. I think it's the latter - when Vito returned home from the hospital, he was visibly upset that Michael killed McCluskey and Sollozo, and in the scene preceding his death, Vito tells Michael he never wanted this life for him.