1. Jerec was an Inquisitor.

https://starwars.fandom.com/wiki/Jerec_(Inquisitor)

2. The Inquisition was very much a thing in Legends.

https://starwars.fandom.com/wiki/Inquisitorius/Legends

3. To answer u/Mpetey123 some of them did know who Vader really was.

Malorum [later the first Grand Inquisitor] initially worked as an Imperial Intelligence operative, and along with Director Armand Isard, visited the Jedi Temple in the aftermath of Order 66. There, he saw a hologram of Emperor Palpatine addressing Anakin Skywalker as Lord Vader. Thus, Malorum was one of a few in the Empire—along with Prince Xizor, two decades later—who knew Vader's true identity, although Vader and Palpatine were unaware of this fact.

Following the incident, Malorum was plucked out of obscurity by Palpatine, who revealed that he was a Sith and placed Malorum in charge of the Inquisitorius