The term exists long prior to Vatican II to refer to a time in between the election of a major Bishop or of a Pope.
I would also like to point out that Sedevacantism is made up of pretty much 000.1% of Catholic laymen, who had no power, or authority in the church. Sedevacantism pretty much damages not just the church, but also the Traditional Catholics who are loyal to the His Holiness Pope Benedict XVI.
Sedevacantism claim that the Pope has lost his office, or was not validly elected to begin with.
From the website;
http://athanasiuscm.wordpress.com/2008/ ... -it-wrong/
It briefly sums up Sedevacantism in this manner:
Invalidity of all ordinations and consecrations since 1968- this is a more recent position, namely that all Bishops and priests ordained since the change in the ordination rite in 1968 are invalidly ordained and therefore their sacraments are invalid, Mass, confessions, annointings, and confirmations. Who has the valid sacraments in the Church then? Why, they do of course. How convenient.
Invalidity of the Novus Ordo- Sedevacantists believe the Novus Ordo is invalid, always.
Feeneyism-though Fr. Feeney was never a sedevacantist or a schismatic, sedevacantists have made one of their doctrines the same unbending interpretation that Fr. Feeney gave to Extra Ecclesiam Nulla Sallus, calling anyone who does not agree a “salvation heretic�. Unfortunately that list would also include St. Thomas Aquinas, St. Pio (Padre Pio), Fr. Reginald Garrigou LaGrange, St. Alphonsus Ligouri, St. Therese of Lisieux, St. Teresa of Avila, Domingo Banez the spiritual director of St. Teresa of Avila, Bl. Pope Pius IX etc. This of course doesn’t bother them as they assert anyone who disagrees with their interpretation of the doctrine is a heretic, much as Protestants do. Fr. Feeney at least practiced what he preached and brought thousands into the true faith by his zeal to convert them. Other than diatribes on blogs, what do Sedevacantists do to win souls for the Church?
Invalidity of Vatican II- The Sedevacantists also claim that Vatican II contains unmitigated heresy and can not be reconciled in any way shape or form. Therefore, for them, any Catholic who adheres to the council is an apostate.
Invalidity of Marriages- Because Sedevacantists believe that priests in communion with their Bishops are not real priests, nor their Bishops real Bishops, they can not have standing under canon law. Therefore, your marriage, my marriage and anyone else who was married by a priest ordained in the 68 rite or by a Bishop consecrated according to that rite can not have jurisdiction, therefore your marriage isn’t witnessed by the Church. Who has the jurisdiction? Well, they do of course. How convenient.
When it all began- As a rule of thumb, most sedevacantists believe that Pope Pius XII was the last true Pope. They believe that John XXIII was not elected properly, or that he apostasized, and that every Pope from that time to the present (which includes Pope Paul VI, John Paul I, John Paul II and Benedict XVI) are anti-Popes and that there is no real true Pope.
Sedevacantists hold these positions, and many more.
It is pretty much another Protstestant group now, who mis-interpret Church doctrines, nor understand it.