I think you’re really talking about two related but distinct questions. What is death being one, and what measures should we take to prolong life or hasten death being the other.
What constitutes death is usually argued about under the umbrella of an overarching misconception—that death is a discrete moment, that life winks out suddenly like a light bulb. It really isn’t like that. From a biological point of view, dying is a process and it is ultimately arbitrary what point one chooses to change the label from alive to dead. Especially now that technology can keep some vital functions going, or substitute for them (breathing, heartbeat, etc), the death of the body is a process not a momment. And I suspect the irretrievable end of consciousness, which is what we care most about for humans, is a process not a moment as well. In his book Consciousness Explained, Daniel Dennett does a great job taking apart the, I thbink, mistaken notion that consciousness is unitary in space and time, that it can exist at one moment and not at another. It seems sort of like this to us subjectively (though when was the last time you were able to say precisely at what moment you fell asleep?), but in reality consciousness is an artifact of many processes happening in the brain in parallel, so there is no physical center and no possiblity of a single moment in which it goes from on to off.
More to the point for you, I think, is what is the clinical or legal defintion of death, and what are the moral implications of the definition we settle on. Not breathing and not having a heartbeat have long been definitive, because these are discrete identifiable milestones in dying, and for most of human history generally they constituted a point of no return. The persistence of brain activity for some time after this “death” was unknown, and somewhat irrelevant since there was no restarting the heart or respiration. But cases like Karen Ann Quinlan arise because we both know more about the gradual process of dying and because we have pushed the point of no return back farther, sometimes much farther. Exactly when a person’s personality or consciousness is irretrievably lost (or when it can be said to have come into being, at the other end of the life cycle) is no currently knowable. Even people in non-responsive conditions such as coma have returned to some recognizable level of function after years, though this is rare.
The debate is complicated by our belief that death just shouldn’t happen. Especially in America, we just can’t accept it. And even though most people claim a religious conviction that physical death isn’t a real or permanant end to personality or consciousness, it’s amazing how hard most people fight against it. As a vet, I see and cause death regularly. I believe, as do most of my clients religious or not (though certainly not all) that witholding aggressive life support, or even deliberately causing death in the face of a very high probability that death is imminent anyway, that the restoration of a good quality of life is impossible, and that the patient is suffering or completely unaware, is morally appropriate. But for some reason, we apply a different standard to humans. All efforts to prolong life must be made regardless of the likely outcome or the suffering or dysfunction of the patient. Why? Historically, it has been argued that God wants us to struggle through whatever suffering He offers as a sign of our faith and as a test of our use of our free will. That still sways a lot of people, but not as many as it used to. So the folks who believe that now try to couch their opposition to ending life support or physician-assisted suicide in terms of the uncertainty of the ultimate outcome, the potential for abuse by unscrupulous relatives or doctors, etc. Legitimate concerns in themselves, but far overblown as a real threat in order to oppose something that ultimately the opponents feel is morally wrong for religious reasons. Not believing that suffering is beneficial to our souls, I believe people should have the right to opt out of it when technically possible. And since death is real and inevitable, we should value life and the aspects of it that make it meaningful to us rather than just oppose death on principle.
So, long answer (Occam hates it when I do that) to a short but complicated question. Basically, I would answer your specific questions
1) Did she die in PVS? Depends on what you mean. Her personality was, most likely, irretrievably lost, and I think that is a more meaningful definition of death than one strictly relating to organ function, but again the definition is arbitrary and dependant on what you think matters.
2) If she was alive, was it ok to pull the plug? Well, if by alive you mean measurable organ function but no possibility of a return to consciousness or some significant apporximation of normal function, then absolutlely it should be permissible to end what’s left of her life.
3) Pulling plug=murder? Morally, no way. Legally, depends.