I know this is anticipating technology that wasn't around
		
		
	 
Actually, advances in science have made Pike's condition more plausible for the 2010s audience, not less...
Back in the 1960s, few in the audience would have been aware of the wide  range of brain damage types and cognitive disorders. Even the writers  would have been thinking in terms of "His mind is locked inside a  damaged body we can't repair". But from the viewpoint half a century  later, it's pretty simple to assume the opposite: "His damaged mind we  can't repair is locked inside a body we can repair (but to what end when  he can't use it anyway?)".
That is, Pike would not suffer from lack of vocal cords. Nor would he  suffer from being a total vegetable. But beyond these 1960s  alternatives, there's the plausible one where Pike has lost the ability  to use language. Yes, he can be positive about things, or negative. But  no, he can't form a coherent phrase to describe these feelings, or even  an incoherent one. His chair can guess on his positiveness or  negativeness and relay that to the outside world, but his thoughts will  not be formulated into words, or even into what exists before a word is  formulated and relayed to the assorted speech organs.
Not even a mind meld would necessarily help, then, because Pike isn't  merely unable to say out loud "I want that glass of water", he's unable  to 
think it out loud, either. That's still completely within McCoy's  parameters of "His mind is as  active as yours and mine, but it's trapped inside a useless vegetating  body". McCoy would simply equate the body's (the brain's) inability to  provide language with the body's inability to provide motion, or  heartbeat, or digestion. The mechanistic trouble with turning thought into language would not mean an enfeebled mind, it would merely be another variant of being physically crippled.
There'd be no point in having a voice synthesizer in the chair in that  case. All it could do would be to put "yes" and "no" into more complex  sentences, arbitrarily chosen and not reflecting the thoughts of Pike  any better than simple "beep" and "beep-beep".
What the chair could use, I guess, is a pair of manipulators. Pike is  apparently capable of controlling his movements (and we can assume the  chair is capable of cross-country movement, despite its Dalek-like  design), so he might derive satisfaction from being able to manipulate  things, too. But perhaps such aesthetic-psychotherapeutic improvements  would come later? It's been "months" since the accident, but perhaps a  few more months would see Pike given a proper android body?
Timo Saloniemi