Presidential Succession and the Vice Presidency

The first cabinet member to take the office of President in the line of succession is?

Who was the first V.P. to move to the office of the President?

What made the practice of the V.P. taking over for the President? 

You have to have the same qualifications to be V.P. as President.

Which V.P. would define the 25th Amendment?

Where can we find the tool Congress uses to fix the succession following the V.P.?

Who can override the Presidents written notice to Congress that he can resume his duties?

If the officed of President becomes vacant then Congress appoints of new one.