From the article:
Steven Speray: "St. Vincent Ferrer, a Dominican highly educated in the Faith, became a sedevacantist by his own judgment against his friend, Pope Benedict XIII. He knew his Faith, and he put it into practice. St. Vincent Ferrer, pray for us!”
John Lane: "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."
In a sense, during that time, everybody relied on "private" judgment to determine who was and was not a legitimate pope. It's not unlike the situation today, where people have to decide whether to stick with the Conciliar Church, align with an FSSP, align with an SSPX, go with the Resistance, or go with the sedevacantists. There are in fact many similarities here.
It's only during a time of Universal Peaceful Acceptance that the popes are accepted by more than private judgement, that there's a certainty of faith regarding who the legitimate Pope is. But the Great Western schism is a perfect example of where this UPA was lacking, and it is my opinion that it is lacking today. Absent UPA, the only alternative is private judgment.
So those who hold there's no UPA today can rightly make the assertion that St. Vincent judged the status of the papacy based on private judgment and that a similar situation applies today. Given the confusion at the time, he could just as well have concluded that none of them were legit as having concluded that any one of them were legit ... and all of it would have been with private judgment. Whether or not St Vincent concluded that the See was vacant is not actually all that relevant. Even if he concluded that X, Y, or Z was the legitimate pope, that was on the basis of his private judgment. So this is really the wrong argument to be having. In other words, I don't really care about this debate. Really the only argument is whether UPA applies today, and it is my opinion that it decidedly does not (for reasons I've articulated elsewhere).