Well, Memory Alpha says that non-Vulcans have used the neck pinch with varying degrees of effectiveness. While it may traditionally be at least partially psychic in nature, several non-telepathic people/races have been shown to successfully use the Vulcan nerve pinch (Picard, Odo, Seven of Nine off the top of my head). In the Voyager episode Cathexis (Season 1, Epidode 13), after Kes has been attacked by Tuvok's Vulcan neck pinch the Doctor claims that Kes has had nerves in her trapezius bundle ruptured. That would lead me to believe that the neck pinch is used to disrupt blood flow to the brain much like a choke hold would. The problem with that comparison is that a choke hold takes several seconds to render someone unconscious, while the nerve pinch is almost instant. Perhaps using a large amount of force and applying it quickly and precisely, Vulcans (among others) can disrupt blood flow enough and create a drastic pressure difference in the brain that can instantly knock the victim out.
That being said, since we don't know exactly how the Vulcan Nerve Pinch works, we can't exactly determine who/what would be immune. While immunity to the mind meld would imply either a resistance/immunity to telepathic abilities or strong enough telepathic abilities to block/disrupt it; immunity to the nerve pinch could require resistance/immunity to telepathic abilities, or it may require having a physiology that can either compensate for an extremely quick change in pressure within the brain, or a way of regulating that pressure that is quite different than our own, i.e. having arteries that transport blood/blood analogous material run along the spine as opposed to them running along the neck, having these arteries be extremely rigid or armored, etc.
Memory Alpha page on Vulcan Nerve Pinch:
Memory Alpha page on Cathexis: