When plants vegetate, they spread. When people vegetate, they relax. After a long day, if you want to vegetate or exert no effort in your body or mind, try sitting in front of the TV for hours.
The word vegetate has opposite meanings depending on whether you're talking about plants or people. When vegetables vegetate, they proliferate or spread really quickly. When a person vegetates, he remains completely idle. A person in a vegetative state is not able to function, only to vegetate or remain completely inactive.