[Gameplay] Challenging Smite doesnt scale with vayne?

Hi,

First of all Im sorry if that is not a bug and just my miss understanding.

I just had a game with vayne as ADC and decided to try the smite build, when I checked the item description to see how much bonus damage I do it showed zero and didnt change at all the whole game, in the same time in my team I had graves Jung with the same item ( We both build Skirmisher's Sabre - Bloodrazor) and his damage was scaling each level.

Edit: Ok I tested it more, if I just buy the in the start I get the bonus damage (or any part of it will be enough) but if I buy any other item it wont, I will try to record it and upload later.

Hope to get any answer if it happened to anyone else