As of season 4, Vecna does not really have any reason to control the Mind Flayer. Vecna was seen causing plenty of damage by killing Chrissy, his own family, and even attempting to do the same to Max all the while never leaving The Upside Down.
Vecna created the Mind Flayer when he arrived in the Upside Down and identified the Mind Flayer's particles/hive mind as being the missing piece in his plan to gain ultimate power. Whenever Vecna kills someone, he "consumes" them, according to Dr. Brenner; that means he acquires their consciousness and power.
Vecna revealed in season four, he was the one who created the Mind Flayer after Eleven (Millie Bobby Brown) banished him to the Upside Down. All this time, the Mind Flayer was Vecna, who was using the entity to carry out his orders and speaking through the creature.
The Stranger Things season 4 finale revealed that it was Vecna who moulded the Mind Flayer, and its spider-like shape is deeper than you think. In Stranger Things, Vecna fashioned the Mind Flayer from shapeless dust into something spider-like - why?
Even more interesting is after The Mind Flayer's defeat, the Upside Down had never lost its strength. Plus, considering how powerful Vecna is, the idea of not sending him before The Mind Flayer was a strategic disadvantage in an invasion, meaning Vecna may have been the true leader all along.
The biggest difference between these two, and the most obvious, is that Vecna was once human, which fans found out by the end of Stranger Things Season 4. It is assumed that the Mind Flayer does not have the ability to think critically or even speak, however, Vecna does.
From the information we got during the Stranger Things 4 finale, we can conclude that Henry Creel is the one calling the shots; in fact, Henry is the Mind Flayer. The Mind Flayer, it turns out, isn't even its own being at all.
Next up, the viewers go on to theorise that One/Vecna is actually Eleven's dad. Yep, told you it was wild. The theory claims that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
I saw a means to realize my potential, to transcend my human form, to become the predator I was always born to be. Vecna reached out to the Mind Flayer, seemingly using his powers to reshape it into the spider-legged monster he had doodled as a child.
According to Henry Creel, he was brought to Dr. Brenner when his mother caught on to his mysterious psychic powers.
What Does Vecna Want? Vecna's goal is to open as many gates to the Upside Down as he can, in order to help the Mind Flayer.
In 4th edition, the Open Grave book shows the leader of the cult of Vecna is a lich named Mauthereign.
Remember also that we have evidence that powers can be inherited -- El inherited her powers from Terri Ives, for example. So it stands to reason that Henry inherited his powers from one of his parents.
When Will was trapped in the Upside Down and captured by a Demogorgon, the Mind Flayer became interested in Will and decided to use him for his own purposes, showing no care for him.
Eleven has a memory during this episode and realizes that 001/Henry Creel (Jamie Campbell Bower) wants her to join him on his evil journey, and she banished him to the Upside Down. 001 then transformed into Vecna, the horrifying Stranger Things villain.
What does the Mind Flayer want? According to what Flayed Billy (Dacre Montgomery) reveals during his final showdown at the end of season 3, the Mind Flayer's current plan is to kill Eleven because she is its one true enemy and the only force who can destroy it.
While intending to get his revenge on Eleven and Brenner, Vecna also goes after victims who have dealt with traumatizing experiences in the past (reminiscent of his childhood and his time at the Hawkins lab).
Ultimately, it is revealed in Stranger Things season 4's finale that the reason why Vecna was killing Hawkins teenagers was to create a massive gate that merges the Right Side Up with the Upside Down - and he specifically needed four bodies to do so.
According to the helpful folks over at the Dungeons & Dragons Forgotten Realms fandom wikipedia, Vecna was once a human king who became a lich (a powerful undead wizard).
Vecna successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
And he saw the possibility for escape when he realized the extent of Eleven's untapped potential. One manipulated Eleven into plotting her own escape, which also involved getting her to remove the device in his neck. (Henry, however, did not tell Eleven the truth when he said Brenner wanted her dead.
He's the infamous Number One, but had his powers blocked and worked as an aide in the Hawkins National Lab. He befriended Eleven at the lab and not knowing Vecna was evil, Eleven restored his powers resulting in the massacre and his expulsion to the Upside Down.
Using past traumas to break down his victims, Vecna brought Nancy to the pool where Barb was killed by the Demogorgon and revealed her mangled corpse. Vecna then taunted Nancy with her own feelings of guilt - that she was responsible for Barb's death and had forgotten about her.
The Demogorgon monster was created by Spectral Motion, a creature effects company from Los Angeles that has previously collaborated with Guillermo del Toro. They just had a little over two months to finish the creature, so there wasn't much room for error.