Yi broken

Greetings, I'm here to tell Master Yi is broken. I even think the champion could be bugged. Why? Let me explain. I was against Yi as Vi, when yi does his little chain attack, he is completely untargetable. I have ulted him in my last game to interrupt his chain and my cooldown starts to count and meanwhile, his chain keeps going with him having no effect. My ult should have stopped him, but with his chain attack, he is completely immune. So I think riot should fix that, because if the chain attack cannot be interrupted, nothing can be done and the game is lost. Really unfair.