What does it mean to be a vegetable?

It can mean any one of several things. First, it could be a derogatory reference to a paraplegic, quadriplegic, a comatose person, or someone suffering from another debilitating condition or illness. Second, it could refer to a lazy person, evoking images of the first case, above. Third, it could mean that you are, quite literally, a vegetable (i.e. a plant).