Before we nerf Kalista and Vayne, maybe we should look at Blade of the Ruined King first.

Something that I've noticed on both Kalista and Vayne (two top tier ADCs) is that they both rush the same item: Blade of the Ruined King. The reason being is that it provides everything both could ever want: Decent AD, Attack speed, Lifesteal, and in addition to those stats, they get a current% HP on-hit and a solid active ability. While other carries generally have to build Infinity Edge for their power spike, Kalista and Vayne get as good a power spike from BoRK than ADCs get from IE, only faster and with more immediate benefits from it. This is actually pretty problematic in that:

• Infinity Edge offers no sustain, just damage, and is extremely expensive to build (3800g), understandably because again, it's generally the biggest DPS boost to an ADC, especially early game.
• Blade of the Ruined King offers sustain as part of it's build path, just as good damage, and is cheaper (3200g). It gives Vayne and Kalista the biggest boost in DPS due to it's varying stats (And also lets them duel other carries very well, who didn't reach their power spike as fast and couldn't get lifesteal without further delaying their power spike.)

This isn't just true of Kalista and Vayne, plenty of other champions that benefit from building BoRK early on get a very good power spike from it. Despite what sounded like a late game item for dealing with high HP tanks, it's better used early game for when no one has good armor.

One thing that could be done (for starters) is make the current HP% scale with level (so it's only 5%/6%/7%/8% at levels 1/6/11/16 instead of 8% at all levels) Would help reduce the early power spike that rushing this item tends to give.