Assuming Vanessa is still tripping about this being an unholy hallucination, and thinking this might be some kind of test that she's expected to fail, what would happen if a less-than-fake séance holder brought some real medium powers to the table?

Probably not trying to use herself as a vessel (not intentionally, anyway), but a tool like tarot or something else she can get her hands on. Would she be successful? Would it be safe for her, or would it set her up for something she didn't count on? Or would she just be unsuccessful at divining anything or anyone?