Vayne as the iconic true damage carry has been a problem to balance or deal with, only gated by longer range adc harassing her out of lane in more organized play. Giving more true damage to Yi seems like it would make for the same problem.
Even if he isn't doing well, this seems like the completely wrong direction to take. He's already bursting squishies with an on-hit build thanks to his alpha strike damage, guaranteed hit(s) afterwards and the bonus damage from e taking another 3rd of their live.
If he isn't doing well, why not give him more bonus damage to minions and monsters on alpha strike so he can get to late game quicker?
Without being able to shred any- and everyone within a second of invulnerability and half a second hitting them.