Student or Learner
Subjunctive is used to express contrafactual, unlikely situations.
"I wish I were a vampire"
"If I were a vampire.."
We don't say, "If I am a vampire.." because it's not a possible situation.
However, it's possible to say:
"Imagine I am a vampire.." (as if it's possible)
"Assume I am a vampire.."
Is the subjunctive used only in "if" conditional and wishes?
What you said certainly make sense . I also use it that way, but those people who use it otherwise as such:
"Imagine I were a vampire.."
And that confuses me.
They might be associating it with things like 'Supposing I were/was a vampire'
My opinion based on my speculation is this:"Imagine I were a vampire.."
"Imagine I am a vampire.."
Both sentences denote a hypothetical situation (we know there are no vampires in our modern society), but the English language affords the opportunity for us to decide whether we want to emphasise it ... or not.