OK so let me set the record straight about this.
There are many individuals who become Sith after being a Jedi for sometime. And there are many individuals who leave the Jedi Order and become rabid, in a sense. Dark Jedi.
There are also those in which a individual never undertook Jedi training who were simply always a Sith, like Maul, Palpatine etc...
A Dark Jedi and a Sith are not the same. Darth Vader is the Dark Lord of the Sith. Not only in title only, but in mind and thought.
If anyone defects from the Jedi Order and practice the Force using their anger and hostility then they are Dark Jedi, but that does not make the Sith Dark Jedi.
Dark Jedi are imbalanced, semi-trained, rouges who have never had any affiliation with the Sith Order. In others words, a individual who doesn't want to devote himself to the Sith tradition and just wants to dabble in the Dark Side.
If someone with force-senstivity were never Jedi or Sith, wandering the galaxy in a group or by themselves does not qualify them to be Dark Jedi. Just people who have no real knowledge of the Force and use it in a very unintelligent, unlearned way.
A Sith is a individual who embraces the way of Sith under the supervision and instruction of a Sith Master. Darth Vader despite him making a wrong choice and being stuck with it for decades, didn't grumble about his agreement to Sith, especially after we move further away from the events of Revenge of the Sith. There was still a lot of Anakin post ROTS but it didn't last. Only a few years. Afterwards he just embraced the Sith fully as he had nothing else to live for but his rage and allowing the Dark Side to fuel and to replace the void of Anakin Skywalker. Vader was indeed a true Sith. His former life of Anakin was gone. Vader is an entirely different person. All the while he was being trained by the Sith (Palpatine) whether he was initially double-minded about it in the beginning.
I'm assuming you are suggesting that because of the conflict in Vader's final years would disqualify him, or that fact that he doesn't flat out say he wants to amass power and take out his Master would make him less Sith.
But Vader did want to amass power and, in fact, that was his whole agenda with Starkiller (if you still consider him canon) and Luke. It's just that Vader knew that if he had an apprentice to join with him to kill the Emperor they had to be completely sold out and not conflicted because if they failed Vader is fried.
Also Vader pre-suit was in the process of selling his soul to achieve power. He slew his brothers and sisters in the Force to save one, but it really wasn't about Padmé it was about his own greed and lust for power as Obi-Wan states on Mustafar.
Vader then proceeds to tell Padmé that he can destroy the Emperor and together Padmé and himself can rule the galaxy.
Those are indicators of a individual who has become a Sith Lord. Vader, although very new, already has devised a rather youthful plan to kill his master and take his place.
This though never left. It was entirely reinforced by Vader's need to find someone capable enough to help him destroy Palpatine. Many of Vader's apprentices failed or flat out ran away leaving him with this constant incognito search for someone else.
Vader also has a had decades to devote himself to his own personal knowledge of the Sith ways, most of it in the Dark Arts of the Sith to try and resurrect Padmé. By default Vader is doing what all Sith do: Become as powerful as possible. And consequently becoming as self-absorbed and selfish as possible. He is studying the way of the Sith, but more to fill his own lust for control and possession, not only just for the power to take out his enemies say like Maul example.
Despite what another post has said, it is a known fact that Vader did want the absolute destruction of the Jedi. In fact in one novel Sidious has to tell Vader to calm down on the Jedi hunting because it was interfering with the task on hand. After Anakin became Vader, Vader was bent on destroying every Jedi the empire could find. Even his past apprentice Ashoka Tano, but not before trying to get her to join him seeing as she would be a good candidate to help him take out the Emperor. (And of course he is not going to reveal all his intentions) its like if He sees someone as a good candidate he gives them a choice to join Him before he kills them, solely based off the Rule of two.
A Dark Jedi is also immensely weak compared to a Sith. They are again, unfocused and unlearned, trying to use the Dark Side without any real knowledge of it. The closest thing to "trained" Dark Jedi are Imperial Inquistors because they are trained just enough to fight Jedi but not enough to stand against trained opponents like Jedi Knights or Jedi Masters, or Sith Apprentices or Sith Masters. They are kind of like weak, expendable tools that are easily disposed of.
