Jinx and Vi are the same person
If you think it about it, it makes sense. Vi and Jinx are opposites in every way. Vi is melee while Jinx is ranged. There hair is also opposite (Long vs. short, Pink vs. Blue). Nether of them know their parents, and they are hinted to have a "Sister".
This theory work is based on the starfish, cut one in half, two grow back. What if humans could do it? Would our evil and good natures be divided into our two selves?
My theory: That the person who they used to be were creating a machine to clone people. It went haywire and divided Jinx/Vi into three parts. Each part got a different aspect of the person she was. Were is the third person you ask? Behold the missing link! Riot has a champion planed who is a combination of both Vi and Jinx but bring their own style to League (and properly is a mage/assassin type).*
*Note: This is 85% speculation, I came up with this in the bathroom while reading about Vi and Jinx.
*Also the first time I typed this, my laptop crashed. Draw what ever conclusions from that you want.