I think we're out of the spoiler period now. (If not, edit me.) Re: the question about what Vader meant - Vader senses conflict in Galen almost from the beginning of the book. And then we find out at the end that Galen had been holding Juno in his heart as a 'single bright spot.' Maybe Vader finally realized that Galen had divided loyalties and knew that Galen could not help him defeat the Emperor without absolute devotion to the Dark Side.