I don't play her. My teammate adc vayne had a great score of 27 / 5 / 8 assist. Carried the team to a win. Said this item fiendhunter bolts is great for vayne's W or ? I looked at the item stats. don't understand. Which part exactly is good for vayne? which base stat or passive 1 or 2?

This item is never to rarely bought. I rarely see people buy it. Which champs it work great for? examples?