A vestige is clearly not mental, the OFA vestiges are capable of taking over over Deku's body and making him go out of control, we've seen that before when he fought Shinso and unlocked the Black Whip, not to mention their prescience itself can be felt, Nine felt that the presciences within Deku letting him know that he had multiple quirks, the difference is that AFO wants to take over Shigaraki's body himself, so he's pushing to take more control than the OFA vestiges are, hence the cracks on Shiggy's body, also I'm pretty sure that the AFO face within Shigaraki is just metaphorical and an illusion, I don't think Shiggy has a literal AFO face inside of his head