His main weakness is not any particular set of traits, but a constraint imposed by his location: the Chaotic Quest entrance is a portal rather than a staircase, and Vecna is covetous. Vecna will therefore readily warp to you, but will not teleport away to heal.
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.
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).
After learning how Victor Creel was able to evade death back in 1959, Robin and Nancy deduce that playing the victim's favorite song can help them escape the trance that Vecna places them in, with music and happy memories being the only way to keep Vecna's fatal curse at bay.
Vecna's curse refers to Vecna's predominant method of murder and torture. Using his supernatural psychokinetic abilities, Vecna can probe and influence the minds of others. Motivated by a cruel and misanthropic philosophy, Vecna targets particularly traumatized, mentally ill or insecure individuals.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
Vecna draws power from sad and angry memories and that is the main reason he targets traumatized teens. Those teens are a well of power just there to be absorbed. This is not just a theory, Vecna himself told Eleven about the power that angry and sad memories hold.
Connecting to the Mind Flayer's hive mind allowed Henry Creel to become the version of Vecna that audiences see in Stranger Things season 4, controlling the Demobats, Demodogs, and Demogorgons across the Upside Down and the real world.
Vecna lives in the Upside Down and preys on people's past traumas and guilt. The monster curses its victims, making them relive their trauma in progressively more gruesome ways until it violently kills them.
Then, as he was falling through the Upside Down, One became the creature Vecna after lightning struck him in such a way that it changed his skin and made him into the monster he is now.
Vecna's obsession with time appears to be linked to his hatred of humanity. He views time as a human imposition on the natural world; an attempt to impose order on nature.
In simple words, Vecna needs a total of four gates to take over Hawkins and rule it. Now, you must be thinking, why is Vecna killing people if he can take over the complete Hawkins? Well, in one of the previous episodes, it was revealed that with each person Vecna kills, he creates an opening to the real world.
Dr. Brenner (Matthew Modine) clarifies that Vecna "consumes" his victims, as he isn't interested in simply killing them - meaning Vecna takes everything about and from a person.
Realizing he had tremendous psychic power, he haunted his family with visions before ultimately killing most of them. His father was framed for the murders and locked away in a mental hospital as a disturbed serial killer. Henry then found himself in the care of Brenner, who decided he wanted more kids like the boy.
This also perfectly explains why Vecna's Upside Down spot is the Creel House. It is the place where he discovered his “purpose” and he can hang out there with his deadly spiders and gain strength from taking lives.
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.
Brenner. The episode culminates in Eleven unknowingly freeing One from Brenner's control, allowing him to unleash his true power on the staff and children in the lab, killing everyone except for Eleven and Dr. Brenner, who is only knocked unconscious.
He had natural telekinetic abilities--much more powerful than anything else we've seen. He practiced using his powers, killing animals, accessing memories, but all the while pretending he was normal to his parents.
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."
At the height of his empire Vecna was betrayed by Kas. Kas managed to destroy the Archlich before his own death, with the Sword of Kas, a lethal blade of the Undying King's own make, leaving only Vecna's left hand and left eye behind. Vecna eventually returned and by 581, had risen to godhood.
In the Stranger Things season 3 finale, Vecna was able to manipulate the Mind Flayer and effectively steal Eleven's powers, which allowed him to eventually defeat Eleven and carry out his plan to rule Hawkins with the Upside Down.
When Will steps foot back in Hawkins, those strange feelings start to creep back. He can feel Vecna's presence, though the creature was severely hurt when Steve (Joe Keery) and Robin (Maya Hawke) set him on fire and Nancy (Natalia Dyer) popped a few shotgun rounds into him.
After taunting her with incessant chiming from a grandfather clock and appearing before her in hallucinations, Vecna eventually kills her in an incredibly gruesome way. First, she went into a trance with her eyes rolling into her head, then she began to levitate before her eyes were removed and bones were snapped.
The episode then cuts to Vecna in 1980s Upside Down, zooming in on his arm, where a tentacle slides away to uncover 001 tattooed on his wrist. So Vecna is 001, who is also Victor Creel's son.
We know Eleven was born in 1971, which means her mom got pregnant with her during the tests, sometime in 1970. A 1959 flashback in the mid-season 4 finale reveals a 12-year-old One (Jamie Campbell Bower). That puts One, a.k.a. Vecna, as 24 when Eleven was born.