We see in Spock that Vulcans seem to have the ability to mind-meld with a lot of species - humans, other Vulcans, lava silicon-based monsters. Is there actually any species a Vulcan cannot perform a mind-meld with (i.e. is there any reference to a Vulcan not being able to perform a mind-meld with an individual because it was a certain species?) I grant that the obvious answer would be a machine/computer, but I would prefer an answer of an actual species (the Borg really are a pseudo-race and are a collection of species rather than a species in themselves for the purposes of this question anyway).
The only species observed to be resistant to mind melds that Memory-Alpha lists are the Cardassians, however, this is attributed to their mental training and discipline. We only see one Vulcan use it on one Cardassian (Dukat), so its effectiveness may vary person to person (initiator and target).
Spock was unable to maintain his meld with a Kelvan. Any species with a super-intellect (Organians, Q, Douwd, Cytherian, etc.) would undoubtedly be impossible for a Vulcan to meld with. It'd be like an ant trying to understand the thoughts of a human. Plus, changelings cannot be read by Betazoids, and I'm guessing that Vulcans wouldn't be able to meld with them either, as their "brain" isn't something that can be readily accessed.