If a human body requires artificial support for respiration, heartbeat, blood pressure and metabolism, at what point does it cease to be “vegetative” and become dead?

At brain death.

Explanation:

If an individual is awake but not conscious of what is happening in the surrounding environments the person is in the vegetative state.Such individual could respond to certain reflexes as eye blinking,withdrawal of hands in response to touch, ability to sleep and wake up at normal sensations.However, cognitive functions and sensory responses which are related to emotions are not responded to.It is different from Coma in which the individual lacks ability to be aware of the environments and  tom be awake or alert.

The vegetative state ends when brain deaths occurs.This is the state when all brain activities are lost permanently. Breathing seized,and contact with the external world is cut.Thus, the functions of the brain which were supported by artificial supports are lost because the parts of the brain which  control breathing,heartbeat ,and blood pressure are dead. Therefore, the life supports can not stimulate them to life