Double PD on vayne or pd and statikk shiv ?
I was wondering when full build, would i be better to have two phantom dancers or just one along side a statikk shiv?
or

I was wondering when full build, would i be better to have two phantom dancers or just one along side a statikk shiv?
or

Looks like you're trying to go with a lot of speed.
So I'd say
for movement (passive allows movement through units), and
for a little cooldown with extra damage.
I wouldn't do either, but if i had to choose between the two I'd pick shiv because the lightning passive is somewhat useful.
the current best vayne build is believe it or not 6 phantom dancers. the next time you play ranked be sure to acquire freelo with this build. gl john
Pick one, if you go both your damage REALLY suffers. Overall, PD is better as an item on Vayne.
pd is if u get ahead early the passive help u easly move through some objects to prevent unit collision, having a 2nd one just massively reduce dmg when u could just some base dmg item. shiv is if ur behind and u don't plan on running through them much.
It depends on what you need in a given match, but I would generally shy away from stacking items with unique passives unless that item is obviously the best simply because of the stats it grants.
Neither.
Currently, there is absolutely no reason not to build a Phantom Dancer on any ADC. Ever since IE nerf gave it 5% additional crit chance, it's been insanely gold efficient. OTOH, I would say that the Statikk Shiv's passive would be more useful as a second Zeal item, rather than stacking the unique passive on PD.
My current standard Vayne build:
Blade of the Ruined King Bloodthirster Phantom Dancer Infinity Edge Last Whisper Zephyr
These are not necessarily in order (except for getting the BotRK first - the synergy is too strong), and I would usually get Berserker's Greaves in there instead of Zephyr until I'd finished the other five items. I feel like two Zeal items is not the best itemization choice on Vayne in general, since there are better items for extra attack speed, and stacking some more AD makes her non-Silver Bolts skills way better than the second Zeal item would. However, if you like to have two Zeal items, don't stack two identical ones.