A sedevacantist is a specific type of schismatic. Normally sedevacantists are strongly traditionalist groups that believe that the papal office truly exists and that popes possess the charism of infallibility, but that the "seat is empty" at the moment. It is distinguish schismatic groups like that from groups such as SSPX, which believe that John Paul II IS the pope but simply believe they have a right to disobey in non-dogmatic matters.
It has been said by some that the large number of Europoean sedevacantists within the bosom of the schismatic SSPX is what really prevents Bishop Fellay from seriously negotiating a reunion with Rome. Too many of his schismatic members would queer the deal.
When Lefebvre undertook his schism, one of the men he ordained a Bishop was Richard Williamson who was known as a sedevacantist while at student at Lefebvre's Econe.