I have heard of Silver Bolts and frankly I'd argue that that is a bad ability as well. Anything that takes up a button should have a reason to press that button or it should not be there. If there's no reason to press the button the skill takes up you're actively reducing the amount of play involved in the character. That said, Silver Bolts does not make Vayne completely useless if you turn it off. Because Vayne is a ranged ADC and every piece of her kit is built around auto attacking she's still strong, if a little under powered without silver bolts, but no worse than any other ADC when they're low on mana.
Only cause of her Q and E alone.
Vi's Q needs a lot of skill and has a lot of usages and i would say this skill is way harder to use than any skill shot cause you use yourself what means you need ot think about your positioning a lot (who blocks it, whom do i hit, will the enemy avoid it, in which direction, ....). Her E is pretty easy to use, but a perfect timing is really hard (to trigger the CD as early as possible).
These 2 skills have way more possible plays than most other skills in the game. That is enough to make up for the W.