You can itemize against vayne

Warden's/Randuin's reduces her true damage output by 15% if she attacks you, and Frozen Heart reduces her true damage output by 15% if you're close enough to her. Together, they add up to around 28%. (This is assuming that by the time these two items are built, Vayne will have enough attack speed that the auto reset from her Q will be negligible)

In addition to items, there are also other ways of reducing her true damage. You can counterpick with a champion with built-in AS slows, like Malphite, Nasus or even Anivia (Malphite's 50% slow hits her especially hard). Exhaust also reduces AS by 30%.

If you have a Malphite with those two items who hits his ground slam on Vayne, she attacks him, and you exhaust her, you can cut her true damage down by almost 90%. This probably won't happen in an actual game but the point remains.

Of course, it's nowhere near as effective as itemizing against regular true damage or regular %hp damage, but you can still reduce it. So stop saying there is no way to itemize/counter her true damage.