[The Saucepan Man]

Werewolves, fell beasts inhabited by dreadful spirits that he (Sauron) had imprisoned in their bodies.

The spirits which inhabited the bodies of Werewolves may have been Maia spirits. But then again, they may have been something different. As far as I am aware, there is no definitive answer.

Christopher Tolkein, in one of his notes I tracked down, speculates that "vampires like Thuringwethil, being thus able to transform into a bat-like creature and fly aloft, were presumably maiar or some such being."

It would be helpful to know where this came from (I was unable to find it in any of my books). But it is, of course, speculation on Christopher Tolkien's part. And, although he may be best placed of all of us to speculate on such matters, even he is by no means certain. In effect, he is saying that Vampires may have been Maiar, or they may have been something similar.

As for The Vampire and the Wolf, I have found that the were indeed Maiar. The were Maiar of Orome.

Are you sure about that, Nirvana II? I can find nothing on this in BoLT2. Could you give a more precise reference please?

It's important not to present speculation as fact. That's how misconceptions start.