On more of a readability issue, it makes it hard to notice her.
As opposed to Poppy, who is in shining gold and is a distinguishable dot coming at you, Vi doesn't really stand out. She isn't an Olaf or a Riven, both of whom you can see clearly as a threat when they're coming at you, without even needing to cast any spells.
It's not like this is some huge thing; when Vi is charging at doing things, you definitely take notice of her. The only problem is her classic color scheme isn't as vibrant and flashy as Jayce, and it could use that. That, or her gauntlets could be just a bit more bigger/have things added to it.
Of course, if there are some particle effects that haven't been added in yet, then this whole point is silly, but I might as well let it be known.