Darius can pull Vi during her ultimate but the damage is still done.

not sure what you are getting at. Are you suggesting that vi being pulled is the problem, or that she still dealt damage?

Post this under the Bug section or submit a ticket. They fixed a few things that could interupt her ult (I don't remember where i read it. It's either on the PBE or in the new patch). Maybe they just missed this one. But post it in the Bug section or submit a ticket so they can see it.