I don't think that Fredo is generally ignored by Vito. Yes, there's one shot when Vito is bed-ridden, healing up his five bullet holes. But I think that from what we see of the hit on Vito is that Vito keeps Fredo with him, and may spend more waking time with Fredo than with anyone else. To carry it back to the novel, this is what Vito did with the young Santino - used him as a general attendant and driver so Sonny could see things close up and learn. Fredo, unfortunately, lacks the forcefulness to advance beyond this puppy stage and so remains Vito's gofer at 30.