Buying 2 phantom dancer's is very usefull if you need more crit and attack speed. But if the enemy team is stacking more armor, go for a black cleaver or last whisper instead.
If the enemy team got a lot of crowd control, go for quicksilver sash.

I take ignite for Vayne, for taking down enemies a bit faster. But exhaust can be very ussefull aswell in 1v1 fights.

I take a point in silver bolts at level 2 for great damage and farm, but if your enemies is playing too agressive or if they got a jungler, take a point of condemn at level 2 instead for survival.