Vader's authority in the Empire was generally superseded only by the Emperor's. However, Vader's authority was also superseded on the Death Star itself by Grand Moff Tarkin (the guy who commanded Vader to stop using Force Choke). Wookieepedia's Death Star article cites the canon reference book Star Wars: Absolutely Everything You Need to Know in stating that Tarkin was the commander of the Death Star. Tarkin was put in charge of the Death Star at the same time he was given the rank Grand Moff:
“We need to tighten our hold over the Outer Systems,” the Emperor continued. “You will be in charge of that, Moff Tarkin. Or should I say Grand Moff Tarkin. ... The Outer Rim is yours to oversee -- and with it, Grand Moff Tarkin, the whole of the mobile battle station project.”
Tarkin, p. 248
Notably, Vader was present for that conversation -- so Vader was fully aware that Tarkin was in charge of the Death Star.
Since Vader was not the ultimate authority on the Death Star, Admiral Motti evidently felt safe enough to taunt Vader in the presence of Tarkin. Furthermore, Motti taunted Vader because the power of the Death Star was obvious yet Vader's power from the Force was not so evident:
Motti: Any attack made by the Rebels against this station would be a useless gesture, no matter what technical data they've obtained. This station is now the ultimate power in the universe. I suggest we use it!
Vader: Don't be too proud of this technological terror you've constructed. The ability to destroy a planet is insignificant next to the power of the Force.
Motti: Don't try to frighten us with your sorcerer's ways, Lord Vader. Your sad devotion to that ancient religion has not helped you conjure up the stolen data tapes, or given you clairvoyance enough to find the Rebels' hidden fort...
Afterward, the Imperials acted appropriately scared since they had seen Vader's power firsthand.