Vayne and Duskblade

She would have to stay invisible for one second in order to proc it each time, meaning that she's losing DPS by trying to activate it.

You have to be unseen for a full second to get the damage, which means you have to use the entire duration of your stealth, which is clunky, and it will overall reduce DPS - the amount of autos you could get off if you didn't wait would be more damage than duskblade's proc.

That doesn't even count the fact that lethality and out-of-combat movement speed are subpar stats for her, and that she has to sacrifice some other item to get it.

Its not that busted. It probably won't even be good on her.