We had this happen in my family.
My niece, for some reason, collapsed several years ago.. her heart basically went into arrythmia for unknown reasons. She was in a vegetative state for probably 2 months, with the doctors advising her parents (she was 30 or so at the time), that she would not recover. After probably 2 months, she did start responding more and more... they had seen some response off and on.
Anyway, for probably 2 years, they tried to get her into a rehab program, with the thought that she would at least be able to live with her dad. They did get her into a rehab program, but I don't know how much they were getting back from her. At this point, she was fully awake, but her short-term memory were shot, she could do some self-care, things like that, but not much.
Anyway, 1 1/2 years ago, something happened again.... they found her non-responsive in bed.... paramedics got her heart started once again, but she had sustained more brain damage. She was not tolerating more than an ounce at a feed, and was slowly starving to death. Trying to feed her more often just resulted in her throwing up what she had taken in. Her body was essentially shutting down.
Her eyes would open, but it was more of a "lights on, nobody's home"..... there really was no response this time. Previously, she would talk to you, respond, and do things.... she had been doing really quite well, recognizing her sons, talking to them, playing with them, and such. We visited her that summer before, and we all had a good time talking with her, and showing her my new grandson..... she loved babies!
Her parents (who by this time) had medical power of attorney, had to make the very difficult decision to withhold food and liquid.... they called in hospice, and at that point, just made her comfortable. If she seemed in pain (they can recognize that, by the way a person responds to things), they gave her morphine. She passed away 2 weeks later....
Her parents DID try, and didn't listen to the doctors at the beginning...she did recover to a point, and probably had a pretty good quality of life, even though it was in a nursing home..... but when she collapsed a second time, and sustained further brain damage, and wasn't tolerating feeds anymore (via GI button), her body was apparently telling everyone else that it was time to let her go.
It was a very difficult decision to make, compounded by the fact that her mother lived several states away.. (her father lived in the same area, so he had the medical power of attorney).....
What would I do? I have no idea......