Vito sumsvery well it up in his dialogue with Michael. He tells Michael he never wanted "this" for him, but instead it was to be Santino who ran the family business. When he mentions Fredo he simply says "Fredo...well he was....." and his voice trails off. He loved Fredo as a son, and allowed him to be a kind of gofer for him, all the time allowing Fredo to think his job was more important than it was e.g. he was allowed to attend the Solozzo meeting, bt it was never discussed with him beforehand. In the novel Vito was outraged at Fredo's sexual conduct in Vegas, but it appears he approved of sending him away during the NY wars.