Dont know if this is what rito knows this or not but 
with a full 6 stacks on 
actually auto once to put 2 rings of her W on a enemy, and i feel thats kind of broken considering if you auto 3 times you can get a free W proc on anything.
It also wastes a ton of stats on her and requires her to auto six times which isnt as easy as it is on champions like Varus or Kog.
It is a noob trap item on her
That's how rageblade works, nothing new. Even Jax, Yi benefit from this and i'm pretty sure i'm missing more champions. It's pretty hard to get 6 stacks and keep them unless your enemies are not smart and engage on a fully stacked rageblade.
It's intended behaviour. Her W is a single target on hit effect. So it benefits from Guinsoo's Phantom Hit.
Why buy rageblade on vayne when you can buy ie and shiv and 3 shot anyone that comes within your range.
It's what you build on Vayne to gloat how fed you are because while it has synergy with her, the standard ADC items are far better.
I think it complements well with vayne, i dislike the full crit when you're against tanks, it just melts them despite having your w as true damage. The rageblade can be so helpful especially in sustaining with the double procs on your auto when you have death's dance or another item. It's an amazing build. Don't look over it, and get raped next time you meet it if the opinions on this is so low.
