In the United States the Vice‐President has few formal duties, and the importance of the position relies almost solely on the fact that the holder takes over the Presidency if the incumbent dies, retires, or is impeached; they are ‘a heartbeat away from the presidency’. The Vice‐President presides over the Senate, and votes in the case of ties. Presidents have tried to give the Vice‐President roles in specific areas of policy, as roving ambassadors, or as heads of ad hoc agencies to deal with domestic issues.
Vice‐Presidents are often chosen, not for the qualities they would bring to the administration, but in order to present a ‘balanced ticket’ at the election, broadening the appeal of the presidential campaign. The balance may be geographical—a Southern President choosing a Northern Vice‐President—ideological—a conservative being paired with a liberal—or another consideration (e.g. religion, government experience, gender, ethnicity).
http://www.whitehouse.gov/vicepresident/ Site of the Vice‐President of the USA.