I think, PresidentSmith609, that it is the other way around - Max is in Vecna's mind (along with all the other people who he "consumed") rather than t he other way around. Remember, when Vecna was talking to Eleven in the Rainbow Room, he told her "they are not really dead - they are here" (indicating his head). And several other quotes by other characters (such as Dr. Brenner) indicate that Vecna absorbs the consciousness, powers, thoughts and memories of his victims. So Max is alive but IN VECNA's mind.

@PresidentSmith609

If that happens it would be an interesting and emotional fight

@Averhoes Even i know about Vecna's talking and Max most likely being in his mind scape. Even i feel it's most likely.

But i presented an alternated theory, What if?....