Fun fact. Roughly 88% of Vaynes damage is physical.
According to http://www.elophant.com/league-of-legends/champions/damage-dealt?region=all&tier=platinum&period=week at least. In plat her physical damage is 106,873, while her total damage dealt is 120,852. That's not even counting magic damage from various other sources too.
Just because she deals some true damage does not mean building armor vs her is worthless, you are still negating essentially 88% of her damage.
basically makes her deal no damage too, along with reducing her AS so that she can't proc the true damage as often.
and previous season's Strength of Spirit maxed out. I never got to try it though, went Sion in a bot game trying to see the max I could get but the other guys ended too quick.
because even if you are the best possible itemization vs her AD you'll just go to your death screen with 1300 true damage on it because being able to itemize against vayne is bad