Vayne, of all champions, should not be an assassin

So Vayne's pretty strong right now, with a 20% pick rate and 51% win rate following the Preseason update (champions with high pick rates should generally not have high win rates). However, that much is (relatively) fine, because that can be solved by tuning down her numbers. The deeper problem with Vayne right now is that she's treated as an assassin, when she's probably one of the last champions who should ever come close to an assassin pattern.

For a fair while, Riot themselves have called Vayne an assassin marksman, and there's some of that built into her kit: she's mobile, closer-ranged than most marksmen, and can turn herself invisible. However, what completely opposes all of this is her Silver Bolts: not only do they deal maximum health true damage, which is tuned to bypass all forms of defense, their counterplay requires that Vayne take time to kill targets, and leave herself open to counterattack while trailing her opponent. You cannot give an assassin access to literally the best anti-tank damage type around. If Vayne can kill any target at a relatively consistent pace, that is fine, but if she can kill a squishy within a single three-hit burst, that is not okay.

If nothing else, part of this issue might be a slight thematic mismatch: now that Riot elaborated a bit more on classes, Vayne comes across more as a hybrid between a marksman and a skirmisher/light fighter, rather than a marksman/assassin hybrid, which is why she has more target-agnostic damage, which Fiora's was likely based off of. She also has the persistent, relatively short-ranged mobility of a skirmisher, rather than the long-range burst mobility of an assassin, and above-average defensive/CC tools for an assassin, which fits a skirmisher pretty well. Basically, she's designed to be a great duelist who kills any enemy in relatively similar spans of time, which means she currently needs to deal less damage to squishies, even if her damage versus tanks remains unchanged.

A large part of Vayne's issues comes from marksman itemization naturally leading to some degree of burst-per-second damage output, but Vayne makes some of those effects worse, namely with her Q and ult. For Vayne to no longer be an assassin, her Q should no longer interact with crit, and her ultimate should no longer give raw AD. All of that just gives her more flat damage. In exchange, perhaps she could have her W damage scale somewhat with AD, and perhaps her ult could give her more utility to let her survive teamfights, instead of damage she doesn't need. At this point, though, she could stand to just have some raw power knocked off, and those changes would push her into a healthier space.

TL;DR: Vayne is not an assassin and under no circumstances should be an assassin, otherwise her W loses both meaning and counterplay. If anything, she plays more like a skirmisher/light fighter and should have target-agnostic DPS, rather than burst that also happens to be good against tanks over time. Her Q shouldn't crit, her ult shouldn't give bonus AD, and if she needs compensation, her ult should give her utility to let her survive or contribute more in teamfights.