Equius is the only person who resists Vriska’s mind reading on-screen… established as a my-blood-outranks-you thing but perhaps it could speak to this?
I’m thinking the blood hierarchy angle might be misdirection… Gl'bgolyb’s psychic attack gets to the warmer bloods first and takes longer to get at the colder bloods (who generally speaking have minds that are more closed off to psychic frequencies, I guess?), but a little while after Vriska and Equius’ exchange it seems to be implied that Vriska’s powers actually work the opposite way; she finds it “difficult to manipulate the mind of the psychically gifted.” (later on, with some assistance, she incorporates sea dwellers into her army of thralls).
so the fact that Vriska has a hard time probing Equius’ brain and then immediately gets counter-probed right after might be taken as suggesting Equius is, in fact, “psychically gifted”, just in a different way to what the reader might have been familiar with at the time? the particular way in which Equius rejects Vriska’s influence - “D –>
I can feel you trying to read my mind“, not just immunity but awareness - reads like an equal coming up against an equal, like when one narrator senses another narrator dabbling in their story