What are the chances of becoming a vegetable after drug overdose?

Why do you ask? They're fairly high, and serious brain damage is one of the risks. If you are considering suicide, please call 911, your family physician, or some other person who's wise and trustworthy. They can help you get out of impossible situations, and help you find the inner strength go live. 'You can find something better than death anywhere', as the old take says. Best wishes.