Vayne tears to anything with it. You know,
Ult+Tumble+1= A dead target within about 1 second

This item effectively boosts your damage by 200% (with no CD Runes) for 3 attacks. Slap on an IE and that's 250%, slap on Lethality from Offense and that's 260% damage for 3 attacks. Considering you will likely have 2.5 attacks per second after using this item, you will deal 3 attacks worth of 260% damage in about 1.5 seconds.

As an example, if you are dealing 150 damage after resistances per attack your damage after activation would be:
150*2.6 = 390*3=1170 damage in 1.5 seconds.
compared to 450 damage over 3 attacks (with no crits).

Pair the item with champs that boost damage for a short time, like Vayne or Xin and this thing will be vary vary strong.

There is no doubt that its really good. I'll have to see more before I say its OP though.

The thing is SoTD is a very strange item. It's supposed to a niche item for situational burst, but it is just really good on everyone because it scales linearly with AD. It also deter you from building any critical chance at all to get the most out of the item, this is good in terms of controlling the power of AD carry but at the sametime it harms the so called melee carries because they don't get much out of it, which I think is the very reason this item exist, to make melee carries able to burst as if they had spells.

Currently this item benefit champions with autoattack augmentations and on-hit, such as Xin zhao and Rengar(OH MY GOD THE Q) massively as well as making ranged carries stronger than they are intended during early game.

I think it will be better off if instead of a 100% critical chance, it multiplies the critical chance you currently have, just to round the power curve.(I'm thinking something like flat 20% + 2x your critical chance)

I would suggest making it melee only also.

Build straight AD and buy this item to not only cover attack speed but also enable crits?

Do want.

Pop SotD with Vayne, enjoy instantly melting your first target.

The biggest weakness will depend on hard CC. If a hard CC like a knockup gets landed after Vayne pops the item and she can't use all the attacks, she will be incredibly weak from losing all the attack speed. And can I say Nasus's Wither will counter it hard?