I don't think percent health true damage is what Vayne needs, what she needs is armor penetration lethality so she can better duel other ADCs but not eat through tanks to a ridiculous amount. Granted, that is an ADC's job, but she does it too well.
Yasuo. The guy just needs some time out, think Azir/Kalista. Because it's a lot better for a champion to be trashed for a while and eventually balanced than to be overloaded and 100% pick/ban forever. It's gonna suck for a bit for those who play Yasuo, but they can eventually play Draft/Ranked rather than always Blind Pick.
Vayne is perfect as is. She is SUPPOSED to be anti tank and one of the best duelists. Yasuo isn't op. It's the build and items that are making him op if he goes tank because of his passive.