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in the case of zero brain activity, you are correct. However, in many cases of injury or blood loss to the brain, the medulla, which controls respiration, heart rate, and other basal, non-voluntary functions, is left intact, while the cerebral cortex, which is where we think, can be completely destroyed. The reason is a difference in the path of blood flow and a longer survival time of the medulla tissue in the absence of blood. Basically, the medulla is the major priority, as it is also the last to go during excessive alcohol intake.

PVS is characterized as someone who has a completely non-functioning, destroyed cerebral cortex, but a functioning medulla, and perhaps cerebellum, which takes care of balance, and something else i cant remember. Other bodily functions, such as digestion, control of body temperature, etc, are largely controlled by hormones.

So, someone in this state can theoretically be a 100% unconsious, but be able to live indefinitely with a continuous source of nutrients and water.

Zero brain activity is simply that; no brain activity, a flat EEG. Organs and physiological functions can still occur due to the brain stem, if that portion is intact.

Medulla:
Function:
Controls Autonomic Functions
Relays Nerve Signals Between the Brain and Spinal Cord

The medulla oblongata is the lower portion of the brainstem. It is inferior to the pons and anterior to the cerebellum.

Brainstem:
Function:

Alertness

Arousal

Breathing

Blood Pressure

Contains Most of the Crainal Nerves

Digestion

Heart Rate

Other Autonomic Functions

Relays Information Between the Peripheral Nerves and Spinal Cord to the Upper Parts of the Brain

What is Coma and Persistent Vegetative State?
A coma is a profound or deep state of unconsciousness. An individual in a state of coma is alive but unable to move or respond to his or her environment. Coma may occur as a complication of an underlying illness, or as a result of injuries, such as head trauma.

A persistent vegetative state (commonly, but incorrectly, referred to as "brain-death") sometimes follows a coma. Individuals in such a state have lost their thinking abilities and awareness of their surroundings, but retain non-cognitive function and normal sleep patterns. Even though those in a persistent vegetative state lose their higher brain functions, other key functions such as breathing and circulation remain relatively intact. Spontaneous movements may occur, and the eyes may open in response to external stimuli. They may even occasionally grimace, cry, or laugh. Although individuals in a persistent vegetative state may appear somewhat normal, they do not speak and they are unable to respond to commands.

What is the prognosis?

The outcome for coma and persistent vegetative state depends on the cause, severity, and site of neurological damage. Individuals may emerge from coma with a combination of physical, intellectual, and psychological difficulties that need special attention. Recovery usually occurs gradually, with some acquiring more and more ability to respond. Some individuals never progress beyond very basic responses, but many recover full awareness. Individuals recovering from coma require close medical supervision. A coma rarely lasts more than 2 to 4 weeks. Some patients may regain a degree of awareness after persistent vegetative state. Others may remain in that state for years or even decades. The most common cause of death for someone in a persistent vegetative state is infection, such as pneumonia.

http://www.ninds.nih.gov/disorders/coma/coma.htm