There's actually nothing in the mind flayer's mind blast that it's mind-affecting. There's also nothing about psionics all being mind-affecting, and there's nothing about stunning that mindless (or immune to mind-affecting) creatures can't be stunned. If you want to put in the immunity, we need to put it in explicitly. However, like the mind-flayer, I'd leave it out. Why couldn't this stun a vermin? Granted, this is a little sticky if they're battling other constructs, but still.
It makes sense that psionic attacks that manifest physically affect mindless creatures.
also, since Mind Flayers are non-OGL, doesn't it make more sense to crib off the psionic power Psionic Blast rather than the illithid special attack. That's a Telepathy [Mind-Affecting] power.