You still aren't getting it. Fighters build some tank, and some damage, yet pure assassins do more damage then them and tanks are tankier than them. This trade-off for using both stats is not broken, and I think its quite balanced. Tanks on the other hand sacrifice the damage for pure tank, Riot gives them more damage in their kit so that they can deal some damage, and in some cases the tanks that do true damage, and god forbid darius, do WAY too much damage to be considered tanks. I would instead call them something like invincible fighters. In the end unless you are playing a great kiting champion going against them is pointless and takes 3-4 members, and since they also deal damage you will most likely lose at least 2 people in that engage.
In your example I am seeing a blatant issue in your pseudo-logic. You believe that the vayne should build something other than damage? The only pure tank Item that is viable on adc's is GA. Other than that getting some mr/damage items is plausable against a strong ap enemy, but against a yi? there is no armor/ad items to get. If she were to sacrifice her damage to get the tank as you said, she would still lose the fight, because Yi has a greater 1v1 damage output then vayne. This is due to your passives giving your true damage and extra ad. Champions whose kits allow them to build tank is something completely different from what I was talking about at all.
Let's give you an example since you seem to like them. You are playing a ranked game, the enemy team has a Darius on it. You are playing Yi and you are both fed. The darius is building pure armor to counter you, and manages to get 5 stacks during the fight. He now has more ad than you, more armor than you, more MR than you, more health than you, is getting tons of move speed from cleaver, and ends up killing you by auto'ing you for the 5 instant stacks and then dunks you. You die almost instantly. Since you think its all about adapting you build tank shred, you get a cleaver of your own, you get a last whisper, and you go full lethality. The problem is you lost the attack speed you had from your earlier build, and now you can no longer fight him. You try to adapt once again and go half tank shred, half attack speed, (your ult isn't enough) you fight him and he manages to still win, even though it was closer he manages to kill you with ult once again. You then realize that his ult can do upwards of 600-1000 damage.
Where is the counterplay? where is the supposed adaption that you speak of? I'll tell you where it is. The counterplay to juggernauts is to ban them. Garen/Illaoi/Darius etc. These champs are always ban worthy, regardless of the meta. Because if they get fed, they are some of the strongest champs in the game, who also manage to be invincible.
If you want a good example of a balance tank in this game, look at maokai. He is the tankiest champion in the game in the current state of things, and yet he does minimal damage. He works perfectly in the meta and is played or banned in almost all high elo matches. That is how a tank should be, a tank should be tanky, but do minimal damage. A supplemental damage to aid your team in team fights, but not enough to deal half health to another champion in one ability.
In the end, nothing is going to change by me or anyone else bringing it up. Riot is determined to have tanks that deal damage in the game. Thematically it makes sense. Having champions that are that much of a force makes sense. But gameplay wise and balance wise, all this does it ruin the game.
This will be my last time looking at this thread, I can see that you won't listen to logic or reason, and I don't care enough to keep talking about this. There is plenty of data and other games out there that support my standpoint, and I'm just going to leave it at that.
Sincerely,
TheRedBaron20