I can't find the proposition, but at least I will define what a persistent vegetative is:
A persistent vegetative state is a state defined by the absence of any detectable conscious activity even when the subject is in a standby state.
The vegetative state corresponds to extensive lesions of the cerebral hemispheres with the relative maintenance of the brainstem functioning. It is therefore very different from the coma exceeded, which corresponds to brain death and only extends for a few hours or days because of resuscitation measures.
The most common causes of the chronic vegetative state are prolonged circulatory arrest (irreversible lesions occurring rapidly when the brain is not irrigated), strokes and head trauma.