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