I don't know if you were joking or not (it's hard to pick up subtle sarcasm online ) so I apologize if this sounds ridiculously robot-like in it's lack of humor. The cultural slang use of the word vegetable to imply unconscious states (tv watching/coma) comes from the fact that plant life is capable of little to no environmental awareness (perception). Therefore, a human who is rendered less conscious by disease or leasure activity can be considered a vegetable or vegging out.