According to the 25th Amendment of the Constitution, if the vice president of the United States dies while the president is still in office, the president has the power to appoint a new vice president subject to the approval of both houses of Congress. If both the president and the vice president die, the speaker of the house becomes president, according to the Presidential Succession Act of 1947.
The Presidential Succession Act covers only the office of the presidency. According to this act, if the president dies or is unable to fulfill his office, the vice president becomes president. Next in line is the speaker of the house, then the president pro tempore of the senate, then the secretary of state and on down the line of the president's cabinet. The speaker of the house and the president pro tempore of the senate were added to the line of succession in 1945 so that a president would not be able to appoint his successor. Successors to the president and vice president are only eligible to assume office if they are natural-born citizens, which means that they were born as U.S. citizens, whether at home or abroad, and not as citizens of another country. They must also have resided in the United States for at least 14 years and be at least 35 years of age.
Though the president has the authority to replace a vice president who dies in office, he is not required to exercise that authority. Of the seven vice presidents who died during their term of office, none were replaced. The office remained vacant until the next election.