Trump is being accused of a crime.
OF COURSE the vote is required. It is all in the Constitution you can neither read, nor comprehend. There is nothing - NOTHING - in the Constitution about a one-sided, perpetual "inquiry". There is nothing in law about a one-sided, perpetual "inquiry".
NOTHING democrats in Congress are doing right now has any basis in law. We know this to be true, else President Trump would be able to face and examine his accusers. It is a kangaroo court, and should be ignored as such.