The Twenty-Fifth Amendment clarifies that the vice president becomes president (as opposed to the acting president) if the president dies, resigns, or is removed from office; and establishes procedures for filling a vacancy in the office of the vice president and for responding to presidential disabilities.
The Twenty-Fifth Amendment:
"Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress."