Here's a little tidbit about Vader's possible involvement. This comes from makingstarwars.net who is actually pretty reliable for this kind of stuff.
"Darth Vader will have action sequences in Rogue One: A Star Wars Story. We have been told that Darth Vader has had several suits of armor built for the film. There is a hero suit which is a direct copy of the suit we have seen in Star Wars: A New Hope (see Darth Vader Dream Team Assembled?), down to the lenses in the eyes. The most interesting part is that stunt suits have been built, which indicates Darth Vader will in fact see action.
The action sequences filmed are unknown, but there was work pitching the different ways Darth Vader could kill rebels on a battlefield with the Force and his lightsaber. The storyboard “pitches” showed decapitated rebels, people being dismembered by Vader throwing his sword, levitating bodies to make human shields, and so on. The art was more violent than what we’ve seen Darth Vader do on screen before.
I wish there were more details on the setting of the ground battle but the most we could get was that it was a smoky battlefield."
Here's a link to the "Vader Dream Team" mentioned above.