11 Being Bit by the Mind Flayer

I was just wondering if my thoughts are correct. When El is bitten by the Mind Flayer (physical form), she loses her powers. So, when it bit her, did it/Vecna gain her powers so he could do the K1llings from the UD? To me, this explains why wouldn't he have done that before instead of trying to find a host where he can use them to do the K1llings.

What is your guys thoughts?