Since the Duffers have said that Vecna leaves Will there for a reason (probably implying that he wants Will to gain powers, so he can begin to control Will even more), my guess is that he showed Will the memories of him being tortured by internalised homophobia to induce so much anger in himself that it ignited his powers. However, that obviously didn't work - Vecna doesn't realise that Will's self-acceptance of being gay and in love with Mike actually is the thing that unlocked his powers and saved everyone. Self-love and queer love saved three people from death, not fear and not hatred or anger.
Now, my next guess is that Vecna is going to try this again. This time, not just with Will's sexuality, but with Will's feelings for Mike, because he can no longer use the self-hatred thing, as Will has gone past that now. Mike is going to be used as a tool for Vecna's torture of Will next volume.
What we also must consider is that:
Will can see through people's perspectives if Vecna is inside their mind. If Vecna is inside Mike's mind, Will can see his perspective. Could be used for a terrible thing or a good thing, who knows.
We have not gotten a lot of Mike's pov yet, though I expect to get even more of it due to the last 10 minutes of Sorcerer in which he kept saying "eyes on me, remember?" and the pov of Will standing up etc.
Nancy's vision from S4 described everything that ended up happening: the world splitting in half, monsters everywhere, her parents being hurt, her sister being hurt and Mike.... well. That part hasn't come true yet, and it's being saved until last for a reason.
The last thing that needs to happen here is Mike's feelings for Will to come into play, to be made obvious to the audience, and maybe his internalised homophobia to not only be used against him but used against Will, in a kind of - see how disgusting he thinks you are? - way, when in fact, it's Mike that sees himself that way. We all believe in a happy ending for our characters, so how can this be solved? I think you know: Mike needs to come to terms with himself as well. In order for a full recovery of Will, and the rest of Hawkins, Mike needs to have that insane monologue that Finn has been teasing for a couple weeks now. Shits gonna go crazy.
