She brings too much utility to her team to have Carry worth damage. Her top lane is actually pretty good, but with every champ there are some you just shouldn't lane Vi against. She isn't locked into jungle, that is just where she shines the most.
I mean, what would happen if they buffed sona or taric so they aren't locked into support.
Taric got nerfed because he could support and solo lane. If they buffed Vi, who is incredibly strong in jungle and top lane, their would be a two patch nerf following it. Her kit makes her best for jungle, her kit sets up for really nice ganks. I feel like if they buffed her carry ability, she would do wayyyy too much damage and her jungling would be too OP (more so than it is already, which is a scary thought)..
That's just my thoughts though.