my understanding of the Dark Side is that to become a master of the dark side of the Force—not just able to use it, but to control it and to bend it to your own will—becoming unquestionably evil is pretty much a requirement.
Does Vader fall under that?
Are you seriously asking if Vader is unquestionably evil?
Vader wasn’t unquestionably evil. He was pretty evil, but “there was still good in him.”
Star Wars seems to divorce students of the Dark Side from their former selves, from the perspectives of both Ben Kenobi and Vader himself (and Luke, now that I remember “It’s the name of your true self, you’ve only forgotten.”) That is, Vader was unquestionably evil—evil enough to hold Captain Raymus Antilles in the air by his head until his neck snapped under his own body weight, among other things—but the good that is Anakin is still buried in there somewhere. Once Anakin was redeemed, Vader was defeated.
If what you were asking was if Vader is able to control the Force itself and to manipulate it to his own designs, such a topic moves out of the films and into books. I haven’t read Vader-centric books like Lords of the Sith, for example, but Sith-centric books I have read (like the Darth Bane trilogy and Darth Plagueis) seem to suggest that to be lukewarm with the Dark Side is to be limited in one’s force abilities or their effectiveness. This is in part why Kylo Ren was only mediocre in his saber battle against Finn and Rey, for example. According to Darth Bane’s take on the Dark Side, Kylo Ren should have fueled his use of the Dark Side by focusing on the pain in his side from the blaster wound. The physical pain should have aided him, not hindered him, were he proficient enough with the dark side of the Force.