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.