Constitution only imposes 2 formal duties on VP's:
To preside over the Senate
And...
Help decide the question of presidential disability.
Replacing the President:
The Constitution contains a scheme to replace a president who dies, resigns, or is removed from office. Here's the order:
1. Vice President
The 25th Amendment talks about filling the VP's spot in case of vacancy. The Congress may nominate a replacement, who must then be approved by a majority of Houses of Congress before he takes office.
2. Speaker of the House
Following those 3, CONGRESS decided that the 15 CABINET DEPARTMENTS will assume the office in order in which their offices were created.
A President who has become disabled may temporarily hand power to his VICE PRESIDENT. If unable or unwilling to do so, the VP and a majority of the CABINET can initiate proceedings to have him temporarily replaced.
1 reason why the VP has a low status throughout history is because the President's elect them to "BALANCE THE TICKET".
But...
3. President pro tempore of the Senate
The President can NEVER fire his VP!
THE END..
Chapter 13: The Presidency
Section 2: Presidential Succession and the Vice Presidency