I would posit that Vriska's inability to scry Eq's brain is classpect related. She's a Thief of Light, so it'd make sense that she'd try to steal information from his brain, or rather relevancy. And by that I mean something she can use for her own gain. Unfortunately for her, Eq can not provide that. He's an Heir of Void. Surrounded by darkness and misinformation. The Void is literally seeping out of him, because Void is often associated with liquids (sweat,alcohol, the sea...).

there is no reason for this not to be the same thing as what i’m saying