Omen + Frozen Heart changed?

So I was just deleted by a and late game as and complained about the fact that I built omen + frozen heart, yet vayne was still doing insane damage to me it was almost as if was not there despite the fact that I counterbuilt her true dmg w.

So we lost... then a teammate of mine says I cant counter build true dmg no sht?

So I had to explain the atk speed slow on the 2 items, I was told that im drunk and they don`t stack and they left...

But I recall reading on the forums earlier that they do stack, was this changed? Ik they both HAD unique passives with different names but he said they both have the same names aswell

Also does e shield not take into account his armor? I used it with just a slither of hp left and the shield almost didnt last 3 seconds.