The thing is, you're comparing one of Tristana's active skills (with a shorter range than her autos) and her basic attacks to just vayne's passive on her W - basically, just her autos. What's the cooldown on Tristana's E? And is it true damage?
You compare vayne's true damage to facing Kogmaw and a tank. If you need two champions to make a comparison isn't that unbalanced?
You comare vayne to kindred after she has been fed, and actually gotten kills already. . . when again, kindred can't go 0/0 and have the same effect, and her passive can be mitigated by armor.
I said 2000 extra HP. By the time Vayne is steamrolling, that is going to be far past 3000 HP on most tanks. Yes, it is about 100 true damage every auto.. which equals about 200-250 damage that any other carry would be able to do to a tank, on top of whatever damage Vayne has already built. That is insane DPS.
And I already said the problem wasn't only her true damage. It was that, on top of her amazing self-peel (which has a stun), as well as a short dash (usually all that is needed with all of the bonus MS from AS items), as well as invisibility during her ult.. precisely when she is at her most dangerous she has the least amount of counterplay.
We talk about true damage because it is very difficult to balance and requires the most attention. It is a lot of power that, on top of the % health cancels out a vast majority of counterplay options. It has to be watched really closely .. you know those 6 spots? For items? And you know how armor and resistances are.. well.. like a big part of the game, and of the identity of many of its champions? And for those that it is, they also tend to build health? Both of those are completely negated by this passive. Vayne, in one (passive!) skill, has insane counterplay to an entire class of champions. Acknowledging the fact that these things exist (counters to entire classes, ie Rammus, Veigar, Zed and Yasuo to a lesser extent), Vayne's advantage is very strong without seeming to compromise anywhere else in her kit. The only thing that seems tweaked back to account for this is her q.. which makes her invisible during her ult, on top of giving her insane damage on top of her W.
Condemn is not only a knockback, it is a stun. Don't like the engage? See ya. Full health and about to take the tower? E R AA AA AA Deleted.
Nasus counters her, but only if she's stupid enough to stay engaged with him and he's fed to the point where he can catch up, catch up again after E, and 100-0 her without her getting out of reach with tumble. And that's a maybe - if she lands a stun and Nasus already used his W to engage and catch up, there's a good chance he's dying.
How is passive 100 true damage on every auto attack not overpowered? You say this like "only 100 true damage" but that's actually insane for what is required to proc it (click) and the slot that it takes up in the kit. It should be nerfed to current health% so vayne doesn't have an execute at level 2.