If neurologist does 2nd ct scan post sca, and states there is severe brain damage, and this person will be in a vegetative state if they wake up, and also states there is no hope or no chance when asked. how sure are neurologist of this fact?

They would never give this diagnosis without being 100% sure. Good Luck

There are no 100% certain situations in neurology, but experienced clinicians express opinions based upon statistical outcomes. Your neurologist is likely to be highly accurate in the prognosis. If you would feel better with a second opinion in this difficult time, do request this.