Remembering from interviews and theorizing a bit about the images of armorless Vader, I can put together these things we already know:

-Not much Vader in the movie
-Krennic finds himself in a situation “they or us” regarding his position within the Empire. And he takes a huge bet on it.
-There’s no Krennic in ANH
-Vader in the Bacta tank
-Vader’s line in ANH (and I remember this line particularly pointed out by makingstarwars.net) : “There’ll be no one to stop us this time”

I believe that early in the movie Krennic will fire the Death Star on Jedha knowing that Vader is on the Planet. In-universe, this is explained by his rivalry with the formal hierarchy of the Empire. Out of Universe, it’s a clever plot device to get Vader (a great screen-thief) out of most of the film, since most of the time he will be just floating in a Bacta tank recovering, only to reappear at the end to get revenge on Krennic (who “stopped him” from recovering the plans) and chase the Tantive.