Regarding papal abdication: Church history furnishes a number of examples of papal abdications. Leaving aside the obscure case of Pope Marcellinus (296-308) adduced by Pezzani, and the still more doubtful resignation of Pope Liberius (352-366) which some historians have postulated in order to solve the perplexing position of Pope Felix II, we may proceed to unquestioned abdications. Pope Benedict IX (1033-44), who had long caused scandal to the Church by his disorderly life, freely renounced the pontificate and took the habit of a monk. He repented of his abdication and seized the papal throne again for a short time after the death of Pope Clement II, but he finally died in a private station. His immediate successor, Pope Gregory VI (1044-46) furnishes another example of papal Abdication. It was Gregory who had persuaded Benedict IX to resign the Chair of Peter, and to do so he had bestowed valuable possessions upon him. After Gregory had himself become Pope, this transaction was looked on by many as simoniacal; and although Gregory's intentions seem to have been of the best, yet it was deemed better that he too should abdicate the papal dignity, and he did so voluntarily.
Full article:
http://www.newadvent.org/cathen/01031a.htm
I would gather by the article, that the College of Cardinals would be the ones that take power in the absense of the Pope being able to fulfill his duties.
If he were to go into a vegetative state, it seems logical they would elect another Pope in his place, because in essense, he would be dead, considering he couldn't make decisions.