His knowledge of demons is confined to several prominent horror movies and he's going to guess that's not really applicable to real life. He also has a hunch that Carlisle won't know what a movie is and he doesn't want to derail the conversation by trying to explain.

"By a vessel you mean they possess someone? So that might just be the body of some random person she's using to walk around?" He shudders, that's a little horrifying. Briefly he wonders if someone who's possessed is just trapped helplessly in their body watching everything or if they're unconscious. For the first time he's a little glad he normally blacks out while being forced to commit atrocities.

"How do you exorcise them? Could we do that to her?"