Why Does Tarkin Outrank Vader?

Judging by their interactions in ANH, Grand Moff Tarkin outranked Vader, but this could simply be because Tarkin was commanding officer of the Death Star.

Is Tarkin higher than Vader?

While Vader may be the most powerful, Tarkin is capable of outsmarting the Sith Lord. Marvel comics is in the middle of a run of Darth Vader comics which follow the Dark Lord in the early days of his life following his transformation from Anakin Skywalker.

Why does Darth Vader obey Tarkin?

It could be that Vader was under orders to work with Tarkin. The Emperor ordered Tarkin and Vader to accomplish a mission together in the novel called "Tarkin." He did that on purpose so they would build a relationship. Tarkin and Vader shared a mutual respect once they realized how badass each other were.