I’m a little reluctant to get further into it, but there are indications of a shift taking place in the understanding of the condition known as “persistent vegetative state.” That’s a term used in the Arizona form of living will. This blog post at Instapundit (but by a guest blogger, not the Instapundit himself) suggests that some people who have been diagnosed as in a persistent vegetative state are in fact aware of their surroundings and able to respond to questions.
It sounds like a developing area that needs further attention, but I wouldn’t say it should dissuade you from signing a living will.
The contents of this blog, this web site, and any writings by me that are linked here, are all my personal commentary. None of it is intended to be legal advice for your situation.