Vecna needed Hawkins and the Upside Down to fall in order to break down the barrier between the two worlds. The series revealed that Vecna is Number One at the end of season 4. Vecna needed four victims to complete his task, and it became apparent in the first few episodes of season 4 that Max was one of them.
The first time that the Hawkins children lost to the Upside Down was in season 4, volume 2. Vecna had been planning his revenge for seven years after the incident. Vecna could be physically merged with the dark realm since Eleven created a gateway to the Upside Down. Vecna needed to use the Mind Flayer if he couldn't leave the Upside Down. For any major television show starring children, the Duffer Brothers' decision to end the season with their defeat was a bold move.
Vecna used the evil in the Upside Down to his advantage.
Vecna's plan to open gates to the Upside Down was revealed in the fourth season of the show. Vecna began to break down the barriers between the two worlds. Vecna wanted to build his own power after some of his attempts to destroy the world failed. Vecna's second attempt to take her proved successful, causing Hawkins to break apart from the four gate points. Max was put in a coma after Eleven tried to save her. Vecna explained his evil plan after tying Eleven up in the Upside Down.
Vecna and Eleven had a mind battle in the second volume of the show. The first three seasons raised a lot of questions, but the end answered many of them. He told her that he was able to realize his potential through the Mind Flayer. Max escaped from Vecna's grasp thanks to the song "Running Up That Hill" by Kate Bush.
The second volume of the season left viewers with many questions, such as why Vecna didn't attack Hawkins earlier. Vecna didn't survive after Eleven was able to tear her away from Max. The Mind Flayer was absorbed by Vecna so he could use it as a weapon. If Eleven brought him down before, Vecna can do it again. Vecna put Eleven in her place immediately.