SilverWook said:

Or he could be the guy chasing after those who stole the Death Star plans, who ultimately screws up, and Vader is called in? We’ll know by December. 😉

"He [is] responsible for protecting the security of the Death Star project
-Wookieepedia

You’re probably right. It seems like his job is to defend/protect the DS, while Tarkin is the guy in charge of it.
And Vader will probably only show up towards the end of the film after Krennic has either died or it becomes apparent that he’s lost control over the situation.
(There’s a shot of Krennic on Scarif pulling out his blaster as he approaches a battlefield, seemingly in some last ditch effort to salvage the situation. Judging from the trailers it seems that Scarif will be the location of the final battle over the DS plans. The footage evens shows Jyn holding a package which most likely is the data-tapes for the DS. I have a feeling that Vader will appear after this scene, where probably Krennic and most of the rebels will die. Also the Vader shot from the trailer shown at SW celebration, from what I’ve read and seen on cellphone recordings, seems to be of him on a Star Destroyer bridge. This is probably the same as the opening of ANH.)