Village Idiot (VI for short) is slang for a player who seems to habitually say or do the wrong things, then not understand the consequences of saying them. Most of the time, this tendency is due to inexperience with the game. Other times, it is due to poor intelligence or social skills on the part of the player. Still other times, the person saying these wrong things really is scum.
Townie VIs are sometimes used as convenient scapegoats by Mafia, who will build a case around the stupid things the VI says as a case for lynching him or her. The other Townies are sometimes easily duped into such a lynch, or agree to lynch them on policy.
Note: Do not mistake the abbreviation VI for the player Vi. Vi will find you.