User blog comment:Deshiba/No counterplay/@comment-6191229-20140516023751

Wait, wait wait wait. What? Vi is nowhere near broken, as using her ult is her thing. Vi is known as a jungler with top-tier ganks. Heck, her lore has her as a police officer! The point of Vi is to fly in with your ult and then try to not die. Yes, after her ult is cast it is unstoppable, but AND...? Just because you can't stop something doesn't make it broken. Vi's ult itself has decent damage, but nothing ridiculous.

TL;DR: If Vi had an ult like Jinx or Lux, a gigantically damaging and crippling one, then sure, being unable to stop it is OP. However, she does not, and her ult by itself doesn't do a whole lot. Being unable to stop that is nowhere near broken.