One thing that really strikes me about the differences between Kylo and Vader (i'm rewatching all of the Stars War movies) is that while Vader is almost certainly a far worse person in so many ways, Kylo is just a repulsive dude in general. Vader carries himself well to hide his own deep trauma and misery, and his appearance is obscured by the grim reaper-esque suit.

Vader only really wants to defeat our main cast because it gets in the way of the Emperor's agenda. It only becomes personal when he realizes that Luke is his kid, and even when he and Luke confront each other in Jedi, they're actually comparatively civil. Vader seems genuinely sad that he and his son will never be able to love each other as a family, and by the time they battle in the throne room, he's already lost the battle emotionally.

And then you have Kylo and Rey, where Kylo seems to have this seething resentment towards Rey just for existing and refusing to entertain his self-aggrandizing bullshit. When Luke tells Vader that he can be redeemed, Vader refuses because he doesn't believe the capacity for good is left in him. When Rey does the same for Kylo, he gets frustrated that she won't bend to his will. He's a truly creepy guy, and I honestly hope Reylo doesn't become a thing, because the way he acts around her really does feel abusive.

(To elaborate, I'm only not okay with Reylo for as long as Kylo acts the way he does. If he finally gets over himself and starts working towards self-improvement, then maybe a bunch of years later he and Rey can start something, but as is, he's a genuinely awful person and I really hope JJ doesn't give in to fan desires on this one.)