So I was on pro builds the other day and I noticed lots of pros building runaan's hurricane on Vayne. I was always told it was trash on Vayne considering that it didn't proc her w. So I was wondering if any Vayne one tricks or any high elo adcs could explain this to me.
I love playing Vayne but im a boosted monkey 
I dont really play vayne but i heard they discovered its good lately so you should try it probably in a normal game
I think main reason is because she lacks waveclear and it gives her decent waveclear. You can still proc your W but only on main target. Besides that secondary bolts from runnan's apply on hit effect such as lifesteal from warlords so theres that too.
Don't take my word though :P I am not really high elo player or vayne main :D
From what I understand, its a lifesteal focused build. Basically between a BotrK, Warlord's Bloodlust and a second lifesteal item, either Bloodthirster or Mercurial, you basically get a high sustain build allowing for riskier plays. It also helps Vayne get back into a fight if she manages to disengage at low health, as you can just clear a single camp quickly and return to your team.
they buy it for waveclear. since vayne. has the worst waveclear of any carry
So I tried it in some normals and if feels like vayne doesn't have the range to pull it off or maybe it's just me.
well it gives crit now
used to only be an attackspeed item very good for on hit builds but it doesnt proc w
but now that vayne's been buffed as far as crit goes its just as good as any other zeal item
it gives waveclear although id argue for shiv over it if thats your main goal, but it does give some waveclear which removes one of her weaknesses.
it also allows to doubledip into lifesteal so that might be the main reason why people take it over shiv
