As for Vader being in Rogue One, you can't ignore him in any film that takes place from the end of the Clone Wars all the way up to Jedi. He's the most bad ass, destructive force (so to speak) in the universe. He's out looking for the base of the Rebel uprising. He and his death squads are hunting down any rumors of Jedi. Any Rebel battle strategy simply MUST be built around trying to be wherever Vader isn't, simply because he's a game changer. I sort of expect Rogue One to be about an operation that is part of a multi-front operation, where a main Rebel force is elsewhere as a distraction, and the smaller Rogue One team is performing the real operation.
As for Han, I think backstory there can be fine. The best part about the anthology films is that they stand alone. If it's not our favorite Star Wars movie, it's not like that carries on for a whole trilogy. In general I do agree that prequels should be avoided, just because it doesn't give the writers as many creative options.