While Star Wars: Episode III, and especially after the destruction of the first Death Star, he was able to get back into the Emperor's good graces through unexpected means. Despite the years of grooming and the fact that Vader saved Palpatine from tasting Mace Windu's blade, his failure to stop the Rebels from destroying the Death Star lowered his standing in the Emperor's eyes significantly. Nevertheless, by the Battle of Hoth, Vader had regained his position and influence as the Emperor's top lieutenant and close confidant.
Kieron Gillen's and Salvador Larroca's Star War: Darth Vader (2015) explores the disastrous days for Darth Vader following the loss of the Death Star and other defeats that significant slowed the Empire's ability to impose its control over the galaxy. It was also a time when Palpatine began to doubt the effectiveness of Vader as his apprentice. As history has shown, if an apprentice isn't effective, it won't be long before Palpatine views them as replaceable. The period that takes place between A New Hope and The Empire Strikes Back is one of the lowest points in Vader's life as an apprentice, as it sees him stripped of his command and made answerable to Force skeptic Grand General Cassio Tagge, all the while learning that Palpatine has tasked the cybernetic doctor Cylo with creating a group of "replacements" to challenge him.
As he's consistently shown since he was a child, people who underestimate Vader tend to come out on the wrong side of history. So rather than skulk or worry in the aftermath of his demotion, Vader gets busy. First and foremost he addresses the primary issue that first got him into the "Imperial doghouse," namely the string of punishing defeats the Empire has suffered. In Darth Vader #3, he obtains information on who is behind a series of "hit-and-run" operations that have been plaguing Imperial supply routes. He than goes on to tamp down the activities of the galaxy's major criminal organizations, including the Crymorah Syndicate, the Hutt Clan, and the Son-tuul Pride. Lastly, Vader proves decisive in helping Imperial ally Queen Trios of Shu-Torun put down a rebellion.
An Empire Within An Empire
While success on the battlefield clearly helps Vader rehabilitate his Sith "cred" with Palpatine, Gillen and Larroca series shows that what is even more important is Vader showing himself to be exceptionally skilled at what he's historically shown no aptitude in, namely being a hard-nosed political operator. To be sure, what ultimately wins the day for Vader is his behind the scenes maneuvering to identify his revivals, then set them up for failure by his hand. It is an exceptional campaign of deceit and treachery that even Palpatine describes as, "an empire within an empire," where one empire operates in the interests of Palpatine, and the other for Vader. Moreover, the breadth of Vader's shadow ops is impressive since his rivals include of the military's High Command, criminal syndicates, those who would replace Vader as Palpatine's apprentice, and most importantly, Cylo
While Vader's machination would be grounds for his dismissal in most other organizations, Palpatine loves it. To be sure, under Palpatine's analysis, it shows that Vader is taking his lessons to heart and fully committing himself to the dark side. That is, under the Rule of Two, while the master and apprentice are partners, they are also enemies. Vader's maneuvers show Palpatine that he could be as devious as himself. While ultimately, that might mean Darth Vader will kill Emperor Palpatine, dying at the hands of such a capable apprentice is a worthy death for a Sith.
