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I know what the Siscoe and Salza article states/quotes.  I would like to see John Lane's article. Saying he is a "practical sede" is not the same as saying one is a "sede".  Like I said above, many non-sedes are "practical" sedes.  In all my years on trad Catholic forums, I have never seen a sede call St Vincent Ferrer a "sedevacantist".  Is it possible I completely missed this?  Sure. But I think it's much more likely that context is/was critical when trads discussed him.

http://strobertbellarmine.net/viewtopic.php?p=5514#p5514

And this from an old article of mine. Not sure which one now, but it summarises the above in relation to the sedevacantist thesis.

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"We should not decide the legitimacy of the popes by means of prophecies or miracles or visions. The Christian people are governed by laws against which extraordinary events count nothing." (St. Vincent Ferrer, De moderno ecclesiae schismate, Bibl. Nat., no. 1470; quoted by Mourret, History of the Catholic Church, B. Herder, Vol. V, p. 133.)

The law of the Church, reflecting the divine law explained and proved by Bellarmine, was codified in the Decretals as the canon, Si papa. It reads:

"Let no mortal have the presumption to accuse the Pope of fault, for, it being incuмbent upon him to judge all, he should be judged by no one, unless he departs from the faith."

Which, of course, is the identical doctrine as was taught by Pope Urban VIII, in Unam Sanctam. We see in the quote from the sainted Thomist, Br. Vincent, two clear points relevant to present controversies:

1. Christians may and ought to "decide the legitimacy of popes" as circuмstances demand, and
2. These determinations are to be made according to known laws.

Now let's see how he applied them in practice.

St. Vincent had often urged Benedict XIII to arrange a double resignation of himself and the Roman claimant, for the good of Holy Church, so that a single pope could be elected who would be accepted by all of Christendom. He presented this to Benedict as a sacrifice - the sacrifice of his true authority, his true office, for the greater good. However Peter de Luna, Benedict XIII, continued to frustrate all efforts at bringing this great event about.

In the mean time men had growth thoroughly disgusted with the Schism. It had lasted almost forty years, and circuмstances were ripe for a solution to be found. The solution agreed upon was a council, to be held at Constance, which would receive the resignations of the (at that time) three papal claimants, or if necessary depose them, and select a new, universally acceptable pope.

In the lead up to the Council St. Vincent redoubled his efforts to convince Benedict to resign, but to no avail. Such was his fame, and his unequalled moral authority, that the entire world was expecting St. Vincent to bring about an end to the great Schism, and after weeks of penance, unceasing prayer, negotiations, threats, exhortations, more penance, more prayer, even more exhortation, threatening, and negotiation, the thaumaturg was at the end of his resources, with no sign of peace or unity. He declared that he would retire for a few days, and then he would give the solution to the crisis. The world held its breath.

When the great day arrived, St. Vincent was seated with an audience of churchmen, nobles, and Benedict himself, and he delivered the most astonishing address that one could imagine; he declared that whilst Benedict was the rightfully elected Roman Pontiff, his ill-will in refusing to sacrifice his rights for the good of the Church had made it clear that he was, in fact, a schismatic. And as a schismatic, he had forfeited his membership in Holy Church and with it his papal office. He was no longer pope. This epiphany was delivered on January 6, 1416, at Perpignan.

St. Vincent Ferrer was a practical and theoretical “sedevacantist,” who "judged" a pope (that is, judged the validity of the claim of a man to the papacy), and found him wanting, and then rejected him. He had never expressed any doubts about the legitimacy of Benedict's ɛƖɛctıon. Nor had he considered his claim doubtful in any way. His case was quite clearly that Benedict lost his membership in Holy Church by schism, and thus forfeited his office. In other words, St. Vincent applied the principles of St. Thomas and of the Fathers; the same principles later presented by Bellarmine, with perfect, and perfectly clear, consistency.

The effect was stupendous. All but a couple of cardinals abandoned Benedict, and the schism was effectively ended. As Henri Ghéon put it, Martin V, elected subsequently by the Council of Constance, was "Br. Vincent's Pope." Martin V, in turn, recognising the divinely ordained means by which his office had been secured and the unity of Holy Church defended, wrote to the saint and offered him "anything he wanted." St. Vincent wanted, of course, nothing but Jesus Christ. Shortly afterwards he was perfectly united to Him in heaven.

I think that Roman Theo, Siscoe, Salza, John Lane and Steven Speray (who I never heard of till now) are all just laymen hobbyists with too much time of their hands, who think they "cracked the code".