I find this interesting...
Bishop Michel Guerard de Laurier was a Dominican theologian who became a sedevacantist. After serving briefly as personal confessor to Pope Pius XII, Msgr. Guerard believed that the vacancy of the Apostolic See began with Paul VI and not John XXIII. I remembered reading way back that he advised Archbishop Thuc that the vacancy did not begin with Roncalli but with Montini, just after the Dominican priest received episcopal consecration from the exiled prelate.
It seems Msgr. Guerard justifiable to make such a statement because unlike most traditional bishops and priests, he was an experienced theologian who helped Pius XII define the dogma of the Assumption and worked with Popes John XXIII and Paul VI likewise.
Though his opinion on Pope John XXIII is held by a few sedevacantists, the vacancy, in your opinion or point-of-view, began with what Vatican II pope?