A sede vacantist is one who believes the seat of St. Peter, i.e. the Pope, is vacant. They believe the Popes of recent times are not valid Popes, hence the seat is vacant.
You will find a few, not many, in France, but most traditionals fall under the umbrella of SSPX. SSPX is NOT sede vacantist. They still believe the Pope exists.
8mm
I think sedevacantist believe that the poep exists, just that he may be excommunicated. Most sedevacantists today seem to believe that the pope legally holds the office, but not morally.