You kinda lost me when you said Vlad is a high skill champion.
A side from that, the main reason why some of them get nerfed alot is because of set reasons.
he does take skill but his damage is absolutely insane and those that has mastered him will delete anyone, even tanks, with very little counter play. Be glad they removed his revive in the process because it was basically a "get out of jail for free" skill and actually have to rely on proper skill management.
one of the most broken champions that can put any champion to quarter health at lvl 1 if you can trade properly and delete anyone at any level, not to mention her invis is a core issue to her entire existence and her damage is absolutely nuts.
Both Lucian and Ezreal wont get nerfed because adcs and have some problems ingame because power spikes and drops are a thing.
The only usual way that Ezreal gets nerfed alot because his ap and ad ratios are nuts has a problem with abusing items.
arguably has enough nerfs and is in a sort of fine place. Only main reasonable thing they could nerf is her root when it hits more than 2 units. The only thing that they nerfed was her on hit build that was rightfully deserve.
desperately needs to be nerfed because his mid to late game is an absolute nightmare to face because his healing and his period makes him a massive issue.
Sej, Karthus, and Xerath are in a decent spot.
Karthus is constantly nerfed because the fact his ult and Dark Harvest exist.
Xerath in the other hand is notorious for scripts.
In the very end, it's all based on how strong the meta is and what is realistically worth changing and which champion needs to be nerfed.