You misunderstand the lore of the show. The mind flayer is answerable to Vecna, not the other way around.

MF will have Will be used to kill soilders.

^^ The Imortal Eldtrich Celestial Demon answers to no one. Clearly Vecna has been shown to serve him. Vecna is comparable to Darth Tyrannus to me, while MF is Sideos. Will, I think, should be Anakin.

TORTURE MIKE TO DEATH SLOWLY! OPEN A GATE IN THE BYERS HOUSE! KILL THEM ALL! KILL THEM ALL!

@Immediate need Well, whatever floats in your head-canon. Go back and watch the show again if you want to know how the characters actually work.

^ I think you must be confused

I mean I could see that happening which would be an unexpected twist

Well first of all from how I understood S4 Vol2 Vecna controls the Mind Flayer and not the other way around.

Also, Vecna didnt really fail his mission. I mean yeah he got heavily wounded but he still managed to kill 4 people