Hi. I think it is right to say that an intransitive verb can't have a passive form and I think the words "sleep" and "vanish" are intransitive verbs.
I think it also is correct to say that for a transitive verb the participle part of what seems to be a passive verb could be an adjective as in "His work is done." Here, I think the participle "done" in what seems (looks) to be the passive verb form "is done" is an adjective.
Going back to the issue of intransitive verbs and what I think are two examples, the words "sleep" and "vanish," would you say these are not correct?
1. He is vanished.
2. He is slept.
I think no. 2 is incorrect, but for no. 1, I think the verb "vanished" carries the sense of an adjective (I could be mistaken) and seems not to be part of a passive structure. If that is an adjective, could that be a valid ground for an exception to what I think is the general rule that an intransitive verb can't be (have??) a passive form?