What is the role of the vice president of the united states?

The vice president is the presiding officer the US Senate. He has to be ready to take over as president if needed. He also takes on assignments that the President gives him and may serve as special representative for the President.
He/She stands ready to assume the Presidency in the event that the President can no longer perform his/her duties (or no longer wishes to). Until then, he/she presides over the U. S. Senate.