blood will out
Bail and Breha tried so hard to instill Padme’s values into their daughter, and they worked on some level because she is idealistic and charismatic and good, but Leia always flourished more as a revolutionary, with an enemy to fight, than she ever did as a politician. She has Anakin’s anger, but she uses hers as fuel to protect those she loves instead of hurting them.
Sometimes Vader looks at her, a child royal using all her wits and strength in the Imperial senate to struggle against a much more powerful force gradually chewing up and spitting out her people, and he’s back in time, nine years old and smitten with a young queen who would do anything to save her planet. (It is this anguish, and the rage it inspires, that stops him from noticing how her own anger sings to his blood, how she so often thinks about taking her enemies and ripping them apart--)
After the destruction of Alderaan, when he throws the distraught princess back in her cell, he can only see her tears and think of that last encounter on Mustafar. He does not see how dearly she wants to slaughter everyone on the Death Star, like a village of sand people, like a Jedi Temple, like a room full of children helpless to stop her. He leaves her there without noticing the pressure on his throat, how her fingers curl into claws at her sides.
When he goes over any information he can scrounge up on Luke, the Force singing with my son my son my son, he sees the towhead of blond hair, the improbable piloting skills, how much he adores the princess, and thinks that is all there is to him. He doesn’t see, or perhaps ignores, his boundless compassion, how he tries so hard to rely on words before violence, his easy charm, how his loved ones come first even at the expense of himself. He sees a young man from Tatooine, trained by Obi-Wan Kenobi, and Vader’s first impulse is to make him more like him -- Dark, angry, vindictive. But in the end, he looks at his son writhing on the ground in agony as a Sith slowly kills him because Luke can’t bear to hurt the people he loves, even at the expense of his own safety, and it’s Padme on Mustafar all over again.
Then Luke forgives him for all his atrocities, like Padme always had in the end, and he dares to hope that she can find it in her heart to do so one last time.
Vader knows better than to hope that Leia ever will; she has too much of Anakin Skywalker in her.