Was Darth Vader technically the last Sith Master?

Question

As we all know Anakin Skywalker a.k.a Darth Vader was Sith Apprentice under Sith Master Darth Sidious (Palpatine). They operated under the Rule of Two, meaning that Vader would have to kill Palpatine to inherit his title and become new master.

Vader of course killed Palpatine, but in the process was mortally wounded himself and died minutes later. Nevertheless, although Vader supposedly rejected the Dark Side and turned back to the Light in the last moments of his life, we never saw him formally abandoning Sith Order or Palpatine's teachings. This is completely opposite to the scene in Revenge of the Sith were he knelt before Palpatine and almost ceremonially pledged himself to said teachings.

Therefore, my question is: did Darth Vader die as a Sith Lord (at least technically) and was he actually the last Sith Master to ever exist, effectively ending the Order not by killing Palpatine, but by dying himself without training an apprentice? I would not discuss theories that Luke could be his successor, effectively continuing both Jedi and Sith in one person, because this seems far fetched.

Anakin becomes Vader:

Answer 1

did Darth Vader die as a Sith Lord (at least technically)

No, Anakin died as a Jedi. That's why he appeared as a Force Ghost along with other Jedi:

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Additionally, Anakin admitted to Luke that he had been saved (i.e. he was a Jedi again) just before he died:

Luke: You're coming with me. I can't leave you here. I've got to save you.

Anakin: You already have, Luke. You were right about me. Tell your sister...you were right.

_{Star Wars: Episode VI Return of the Jedi script}

(Note also the use of "Anakin" as the character name in the script -- he's Anakin again, no longer Vader. It's the first time he's called "Anakin" instead of "Vader" in the script.)

The junior novelization also confirms that Vader became Anakin again when he betrayed the Emperor:

And then, in a moment, something changed. Perhaps he remembered something heard in his youth a long time ago: an ancient prophecy of the Chosen One who would bring balance to the Force. Perhaps the vague outlines of someone named Shmi and a Jedi named Qui-Gon struggled to the surface of his consciousness. The most powerful, the most repressed thought of all could have emerged from the darkness: Padmé … and her undying love for someone he once knew well. And despite all the terrible, unspeakable things he’d done in his life, he suddenly realized he could not stand by and allow the Emperor to kill their son. And in that moment, he was no longer Darth Vader. He was Anakin Skywalker.

He grabbed the Emperor from behind. The impossibly wretched Sith Lord gaped and squirmed in his embrace, continuing to release blue lightning, but the bolts veered away from Luke and arced back to strike the Sith Lords.

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Out-of-universe, George Lucas himself confirmed that Anakin was redeemed:

Which brings us up to the films 4, 5, and 6, in which Anakin's offspring redeem him and allow him to fulfill the prophecy where he brings balance to the Force by doing away with the Sith and getting rid of evil in the universe...

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Even if Anakin had died as the Sith Lord Darth Vader, he wouldn't have "technically" succeeded Palpatine under the Rule of Two because Vader would have had to defeat Palpatine in a personal duel (a duel is the only way to prove that the apprentice has truly surpassed the master). Anakin killed Palpatine by attacking him while he was using Force Lightning on Luke. That wasn't a duel, so Anakin/Vader would not have earned the title of Sith Master according to the Rule of Two. In practice the apprentice does not always challenge the master to a duel (Palpatine killed his master in his sleep, for example), but "technically" a duel is required.

Answer 2

The rule of two is constantly ignored. It's most likely just a good way to keep a potential future enemy in check.

There is no way to know if he was they were the last Sith.

Just going by the new canon, Palpatine (Darth Sidious) broke the rule of two. He trained Darth Maul while he was under tutelage of Darth Plagueis. When Darth Maul was believed to be dead (but he survives), Palpatine took on Count Dooku (Darth Tyranus) as an apprentice. Count Dooku himself, took on Asajj Ventress. Darth Maul took on his own brother as an apprentice. At that time there were at least 5 Siths (3 of which had a Darth title), and Palpatine was already working on Anakin. All of them are confirmed dead or abandoned the order before the end of Episode 6 though. Ventress is alive, and is a former Sith, but as far as we know, but she joined the Nightsisters and abandoned the order.

Darth Vader himself broke the rule of 2 a number of times in the old expanded universe, but that's not canon anymore.

Palpatine intended to create the Rule of the One, where he'd place himself at the helm of many lower Sith. I'm unsure on the canonic status of the Rule of the One, but at least it's pretty clear he constantly ignores the rule of the two.

We can summarize the rule of two is hardly ever observed between Sith, and seems more like a front, an agreement between two parties bent on betraying one another anyway.

Is Darth Vader the last Sith?

From canon and our perspective, yes, although going from the history of Palpatine ignoring the rule of two, it seems weird he drops to a single apprentice by the time of Episode 4 - 6.