If you are a "vegetable" then you wouldn't be suffering. You wouldn't know what was going on.
If you had a painful, terminal disease, then maybe you would be suffering.
A person is more than a body. If they are trapped on body that is in vegatative state, with the brain damaged beyond hope...I believe they can and do suffer in their soul being.
I don't see any reason to keep a person here when they are just as much gone already.
I don't see the point.