I agree with you that vitamins are generally chemical substances that can be harmful to the body. I'm not sure that it's true that heavier people can fast longer than lighter people although heavier people would have greater reserves of fats.
I completely disagree that weakness is a sign for breaking the fast. Other than the return of hunger, indications for breaking the fast tend to be a combination of symptoms in the right context. If weakness indicated breaking the fast, no-one would get past week one.