Lucian:
She was everything to me... but they took her, and with her, they took my kindness, and my mercy. That was their mistake, because now, all I've got left is a cold heart.
Lucian became a man of dark determination, one who would stop at nothing to purge the undead from the face of Runeterra.
Vayne:
By the time she was a fully grown woman, she had become a grim warrior.
Demacia needed a protector, one who hunted those lost to the darkness.
It is said that those who practice the black arts quake when they hear that the Night Hunter is on the prowl.
They have very similar personalities against very similar, generic, evil with very similar emotionally changing experiences.
I see Lucian as having a very dark, serious and determined personality wrapped in light. His clothing is light coloured, his weapons have light colour schemes and his abilities are white, representing purity and innocence. His personality no longer reflects the pleasantness that comes along with white because after Senna's death he changed into a relentless killer. Heck, even his skin colour follows the idea. Not being racist here just pointing out the contrast of colours and how it follows the idea that I outlined. His general visage and personality, I don't think, is akin to Garen or Jarvan IV who have the luxury of royalty, general power and loved ones.
Vayne, I feel, has a calmer, but still ferocious attitude towards the black arts and monsters in general. She reminds me of Geralt of Rivia. Weapons are medieval. She is cold but calculating, she has to be because unlike Lucian, who wields powerful magical relic weapons she relies on a more crude though more accessible weapon type, crossbows.
She has as much determination as Lucian but she does not do her job to avenge her parents, that was probably sated a long long time ago. She does so, more because she thinks it is right unlike Lucian who still mourns the death of Senna and does his work (atm) more for revenge.
Whereas you have Noxus and Demacia who will never get along because their basic fundamental ideologies completely contradict each other they can't ever be in the same workplace or room.
Vayne and Lucian have similar goals in mind, their ideologies don't really contradict neither do their methods.
Syndra and Zed have one goal in mind, more and more power and the rejection of the ideology of balance and harmony. They don't care much for each other's methods.
Whereas Noxus and Demacia, in no sense of the world, fiction or non-fiction, will ever get along with their current ideology. Because fundamentally they contradict each other, they do not approve of each other's methods. You can't place the relationship between Lucian and Vayne with Noxus and Demacia in the same category.
I feel like Riot's lore team don't communicate enough with the rest of the other teams, or the champion creation team don't communicate with the lore team, in some champions, about how they could fully integrate their champions into the world and keep the immersive nature as well as keep the consistency of the lore.