The Duffer Brothers have now finally explained why Max (Sadie Sink) had to be Vecna's 4th victim. In season 4, Max and the rest of the Hawkins Gang had an epic battle with Vecna which ended with Max being in a coma.
In volume 1, Max was trapped by Vecna and made a narrow escape thanks to Kate Bush's "Running Up That Hill." Later we find out that Vecna had to take four victims with each kill weakening the barrier between the real world and the Upside Down. Max ultimately decided to sacrifice herself in the hopes of beating Vecna. Max could not escape this time as she died in the hands of Lucas before Eleven saved her. At the end of season 4, she was in a coma.
Duffer Brothers have discussed why it was necessary for Max to be Vecna's fourth victim. During a recent interview with Collider, the writers said,
"We always knew. I mean, we did debate what exactly we wanted to do with Max, but we knew that she was going to obviously be putting herself in real danger. I'm trying to remember exactly how we landed on where we did with Max, but it was pretty early on, I believe, that we decided that was going to be her fate for the end of this season.
It had to be Max. Because she was targeted, and she had never rid herself [of] this curse. She was just putting on a Band-Aid. She was using, effectively Kate Bush, her music as an armor. So it was very early on. That was the plan that she was going to remove that armor and more or less sacrifice herself to back them in order to give them a chance to kill him."
It remains to be seen what would be the fate of Max in season 5.
Vecna said that when he kills these kids that he's not just taking away their life but he's also absorbing their essence. Their memories, their thoughts, their souls are all stored inside him. So I think it's safe to say that Max's body was saved by 11 but her mind is now imprisoned inside Vecna. The possibly reassuring news is that if she's imprisoned in him and if they can free her, 11 might be able to put her back where she belongs. We shall see though.