Ok i get it, you want vayne to deal a bunch of damage, here's a quick mini rework idea?

Vayne W

Now deals a percentage of her total AD as max hp Physical damage, capping the damage at about 1% per 50 ad, so that if she gets 500 AD she can do the normal vayne thing but at only 10% max hp damage and it'll be physical

Why?

Because itemizing against her, she could counter juggernauts instead of... everything. It would also encourage some lethality itemization, so she'd counter juggernauts and mages instead of... everything.

by making it physical damage you'll allow some counter build options

she has been buffed and nerfed around her W though, so she'd need some compensation somewhere, maybe her base attack speed? it'll let her hit cap easier so she can put more items into AD.