Patch 6.9 brings in two items that synergize well with Vayne's kit:

Guinsoo's Rageblade
Due to the removal of Enchantment: Sated Devourer, its passive has been moved to the new Guinsoo's Rageblade - without farming stacks or the ranged debuff of the old Enchantment: Devourer. This means that every other auto attack (after getting 6 auto attacks) will proc Vayne's W passive.

Furthermore, it will also apply on-hit effects, meaning every other auto attack will apply damage from items like Blade of the Ruined King or Enchantment: Bloodrazor.

The only downsides to this item on Vayne include:

• AP stat useless on Vayne
• Must keep up Guinsoo's Rage passive to maintain maximum DPS
• Odd build-path

Enchantment: Bloodrazor
This item has multiple benefits on Vayne:

• 3% Maximum HP Damage (Does more damage than Blade of the Ruined King to opponents under 50% HP) - nice for shredding high HP opponents.
• 40% Attack Speed - If relying on Vayne's Silver Bolts for damage, stacking provides the DPS needed.
• DRAGON CONTROL - Having double-smite on your team (taking smite ADC or top) will benefit control over the dragon, a crucial objective in 6.9
• Challenging Smite / Chilling Smite - Challenging Smite gives reduced damage taken and bonus true damage, while Chilling Smite gives nice movement speed and a slow to help kite.

The only noteworthy downside of taking this item on Vayne is the weaker laning phase due to the lack of heal (taking smite instead), alongside the odd build-path.

tl;dr
Enchantment: Bloodrazor does more damage to opponents under 50% HP and is much cheaper (accelerates build) - nice to build later in the game to make up for Guinsoo's odd build path. The smite gives peel or damage (chilling / challenging) and dragon control.

Blade of the Ruined King gives a better laning phase and build path due to building Vampiric Scepter, but is more costly (having the price difference of around the same price of Vampiric Scepter. The active gives a similar effect to Chilling Smite, but also does more damage and heals you for that damage.

tl;dr of tl;dr - Blade of the Ruined King for better laning (lifesteal, heal) and dueling (active). Bloodrazor for objective control and DPS (Challenging Smite).