Question about Villains

So I’m aware that since season one they’ve been kind of writing this as they go with a vague outline of the story. In the last few eps of season 4 we get a backstory to Vecna and how he ends up in the upside down.

So just so I’m understanding, they are now saying it’s been him the whole time in those last eps (classic trope when ending a franchise and making the villain more impactful, tidying up the whole overarching plot - for example Spectre in Daniel Graig’s bond series). So when we see Vecna find the Mind Flayer…has Vecna pretty much become one with him or controls the mind Flayer? Or are they seperate? Dustin assumes Vecna is Mind Flayer’s “5 star General”.

Is Vecna the Mind Flayer now too?