Who told Darth Vader that Director Krennic arrived?

Question

In Rogue One, a man in robes kneels before Darth Vader's bacta tank to tell him Director Krennic had arrived.

Who is he, is he some sort of Sith priest?

Answer 1

His name is Vaneé. The Star Wars Databank entry for him simply calls him a "black-cloaked attendant":

A black-cloaked attendant to Darth Vader, Vaneé interrupts the Sith Lord’s meditation in his rejuvenation chamber on Mustafar to announce Orson Krennic’s arrival. After Krennic’s audience with Vader was over, Vaneé advises the director that it would be wise to say nothing of what he’s seen in the Sith sanctum.

His Wookieepedia article also refers to him simply as an attendant. Wookieepedia notes that he was first identified in Star Wars: Rogue One: The Ultimate Visual Guide, which contains the following entry (H/T @Jason Baker):

It is not clear if he was affiliated with the Sith or if he was simply a lackey in the service of the Galactic Empire. On the one hand, Palpatine and Vader weren't recruiting a bunch of new Sith acolytes (due to both the Rule of Two and the need to keep Palpatine's Sith identity a secret), so Vaneé might not have been recruited as a Sith acolyte despite his job at Vader's castle. The canon novel Lords of the Sith suggests that only the Royal Guards could be trusted to know Palpatine's secret (i.e. not even a personal servant of one of the Sith Lords would know about Palpatine's Sith identity):

[Vader] seldom saw his Master so publicly demonstrate his power. And he understood what it meant, of course. There must be no survivors who could bear witness. Only the Royal Guards could be allowed to live—only they could be trusted never to reveal what they’d seen, or even to talk about it among themselves.

_{Lords of the Sith, p. 168}

On the other hand, Palpatine did allow a very small group of people to learn his secret (such as Yupe Tashu, and likely Mas Amedda). It's possible that Vaneé was a Sith acolyte and a member of this very exclusive group of people who knew about Palpatine's Sith identity.