https://crownofmartyrs.wordpress.com/2008/04/05/from-a-biography-of-st-vincent-ferrer/
This is from a biography of St Vincent Ferrer and makes it clear that what he was proposing to Benedict XIII was a condition where he would either resign for the good of the Church or depose himself through schism.
Indeed, which Saint would ever refuse obedience to a pope they considered legitimate?
I am not satisfied with this. I don't think anyone should be. There must be some words by St. Vincent somewhere. This short and obscure biography was written by a Catholic playwright, not someone who is careful with canonical language. Take a look at how he used "treason":
Upon Vincent himself would be the guilt of the Schism if he did not use all the means in his power to bring it to an end — even if it meant treason and rebellion against his sovereign and benefactor. Furthermore, his reason, which never failed him, saw clearly that if Benedict resisted, he would be deposing himself.
It sounds like the author is either suggesting the Saint was willing to sin, or that other people might think he was being treasonous. It could be the latter, but why would anyone think it treasonous if he explained the man was no longer pope? It looks to me like it is just a very loose and flowery way of making the narrative exciting without intending precise wording.
Further, in this quote, it says, "deposing himself", but nobody does such a thing to himself, others do. People actually resign instead, so it is again very loose and sloppy use of words.
Then the narrative mentions, "Benedict XIII was deposed". I cannot believe the author really knows the terminology, but lets assume he does mean that word correctly, it can still legitimately apply to a case where a man who is not a true pope is deprived of what he wrongfully possesses.
I would like to see wording from St. Vincent, or at least some scholarly source making it clear St. Vincent believe Benedict was a true pope who ipso facto ceased to be pope through schism.