We know about when Vader challenged Palpatine for the last time. That occurred when he threw Sidious into the reactor shaft in Return of the Jedi. Yet, there were other times when Vader took his master to task, both directly and indirectly.
Three Times Vader Directly Challenged Palpatine
In terms of direct confrontations, the most memorable one is when Vader hoisted Palpatine above his head to stop him from injecting Force lightning into Luke. Here, we can relive Vader’s glory of defeating Sidious and presumably killing him by throwing him down the shaft. We can count this as Vader’s third fight with Palpatine.
This video provides a quick recap of the two times before this final confrontation that Vader directly challenged Palpatine. The first occurred right after Anakin/Vader’s reconstructive surgery when Palpatine “pulled Vader aside to teach him a lesson.” Sidious wanted Vader to fight him so that he could fuel Vader’s dark side anger and remind him of his subservience to him.
https://www.youtube.com/watch/gdEC__FYWOo
Palpatine knocked Vader down in the first fight and took his lightsaber. This second video (about 1 minute in), provides more insight into this duel, indicating how it served as “a lesson in submission.” It occurred during a time when Vader was still reeling from all that had happened on Mustafar and was not accustomed to his new suit. Vader had “no chance at all” at defeating Sidious.
The second time that Vader took on Palpatine was soon after the Empire Strikes Back. In this second matchup, Vader held his own and even came close to winning the duel. Sidious commanded Vader to stop the fight to avoid embarrassment.
A vast difference in circumstances had transpired by the second duel. As the second video above comments (at 1:56), Vader had now become “hardened, refined” with powers that could potentially overwhelm the Emperor. On the surface level, Palpatine was “testing” Vader, but Vader didn’t “end up on the ground defeated.”
Instead, we could very well walk away from this second fight thinking Vader was the one in charge. The second duel certainly set up his final direct confrontation and victory over Palpatine.
Many Indirect Challenges to Palpatine
Beyond the above three moments of direct challenges, Vader had numerous occasions when he challenged Palpatine indirectly.
Although we could count the Ryloth mission as a direct challenge, it seems to be more indirect because Vader went along with the Emperor’s wishes. During his mission to the planet, Vader prevented Palpatine from killing a Twi’lek girl–certainly a challenge. Yet, Vader complied with Sidious later and wiped out a village on his command.
In the comics, we have plots by Vader to overthrow the Emperor that count toward indirect challenges too because they tended to be based on subversive plans. In the Marvel comic series Darth Vader, Vader forged a secret droid army for this purpose.
As the opening crawl of Issue 5 states, Vader “acquired a personal army composed of battle droids” to secure his quest to find Luke and deleverage Palpatine. But Vader ended up using that army to fight the legions of new apprentices that the Emperor had obtained to thwart Vader’s efforts.
A similar outcome occurred with Vader’s fight on Exogol. This previous may4bewithyou article conveys how Vader took control of a Summa-Verminoth creature in an attempt to destroy Palpatine. Yet, Sidious was again a step ahead of Vader and defeated him before unveiling the purposes of Exegol.
There are other occasions too, which we may categorize as indirect challenges because of the way Vader went about them or the outcomes. Exploring angles to Vader’s attempts to overthrow Palpatine veers into parallel questions about why Vader couldn’t kill Palpatine.
Vader ultimately concluded that he would need to enlist the help of Luke if he was going to pull off a direct challenge to the Emperor. That’s what we saw in the movies, especially with Return when Luke refused to go along with the plan, but Vader succeeded in overthrowing Palpatine anyway.
Vader Challenged Palpatine
We have three concrete times when Vader directly challenged the Emperor. We have many more indirect challenges that coincide with Vader’s plots against Sidious. Based on Return of the Jedi, only one worked.