What you're saying is true, but at the same time isnt really comparable.
Warwick is one of the lowest-interaction Champions in the game, especially since he is a jungler. When he was 4.20 Weedwick, the general practice was farm uninteractively until 6, gank. Repeat until you no longer need ult to win fights.
Comparatively, Vladimir has a steeper learning curve than Warwick (while he's not
, he's certainly harder to play than
) - between managing E stacks, knowing when and how to use your W, and dealing with the early laning and his short-range, high cd Q.
Secondly, Warwick's ease-to-play makes him incredibly accessible as a Champion, meaning that non-Warwick players could easily pick him up during his reign of terror and wreck face, whereas Vladimir's higher difficulty (as well as his much-cited abysmal pre-9 laning) makes him less accessible and thus has a lower chance to be in that bracket of 80-100% of games.
Again, Vladimir isnt super hard to learn, but he's not as easy as Warwick and thus wouldn't have as much of an impact since not as many people would be willing to play him.
Furthermore, on your opinion of nerf-then-buff - there are inherit problems with that. While on the one hand the example doesnt cause problems by being overpowered and impacting games, we get the dual problem of a) not knowing what about them made them overpowered and b) leaving them (and their playerbase) in the gutter.
Why was Warwick strong? The immense synergy of his ult's on-hit and the new Devourer item + red smite, as well as the ease of which he could be played. We know this because he was "allowed" to be overpowered for a little while, and that was what Riot nerfed when he needed (granted they went a little overboard with his W nerfs too, but that's another story).