In Mortalium Animos Pope Pius XI says this

11. Furthermore, in this one Church of Christ no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors

Antipopes were not legitimate successors to Saint Peter, so does this mean St. Vincent Ferrer was a schismatic until the end of his life? Does this mean anyone who rejects or doubts the authority or power of a true pope in any circuмstance is a schismatic? Even during something like the Great Western Schism?

A doubtful pope is not considered to be a true pope, so either accepting or not accepting him is not binding.  This is a fact.  So St. Vincent Ferrer wasn't ever a schismatic despite his material error.

The thing is though, there were multiple widely accepted claimants, and none of them had the universal acceptance of the Church, so it remains to be proven with certainty that Sedevacantists can be covered by the same doubt.  And of course the Sedevacantists could plausibly argue that Sedeplenists, unlike St. Vincent Ferrer, are following a Pope that is not a Catholic, and that that cannot be excused

This question is not referring to sedevacantism, so why do you introduce that topic?
As I understand the question, at the time of St Vincent there was disputation over which one of three men was actually pope. As the question was not yet answered, and St Vincent would be ready to accept the decision when it was finally resolved, he was never schismatic.

It might be more to the point, to phrase the question like this:

"If a Catholic believes that the status of the Chair of Peter is something other than it actually is in fact, and thinks and acts accordingly, is he, then, a schismatic?"

This covers everything --- adherence to a false Pope (thinking he is the true one instead of another claimant who, in fact, is truly Pope), adhering to a papal claimant when the see is indeed vacant, or not adhering to a claimant when the see is indeed filled.  It also covers all the variations on the sedevacantist theme --- sedeprivationist, sedeimpedist, materialiter-formaliter, Siri thesis, Benedict thesis, and so on.

Incidentally, my answer is, "of course not, of course he's not a schismatic".

The correct answer is "it depends."  Certainly someone who refused to submit to Pope St Pius X would be a schismatic.  We are bound to submit to certain popes, on pain of schism.  We are *not* so bound if the identity of the Pope is doubtful, even if our choice isn't the most prudent one.

I see whether the conciliar claimants are truly doubtful as an unanswered question because I think both sides make arguments that are illogical and I see no way to resolve it at the moment.  Even still, even if it was objectively not doubtful, and thus objectively schismatic, I still believe a Sedevacantist could err in good faith, but then, I also think an EO could err in good faith, so like... 

The reason I brought it up is because the question was asked in the Crisis of the Church subforum so I assumed there was some relation

In your scenario, it could well have been, that a priest in remote mission territory would not be aware that Leo XIII had died, and would still celebrate Mass una cuм Leone.  He would not be schismatic, just inculpably uninformed, and acting in accord with the best information available to him.  He wouldn't "refuse to submit" to Pius X, because he wouldn't be aware there is a Pius X.

Then he didn't *refuse* to submit to Pope St Pius X.  He simply had no idea that there was such a Pope, through no fault of his own, that's fine.

What I'm trying to figure out is whether the sedevacantist thesis that the See is currently vacant (and has been for 62 years and counting) is in any way equivalent to the St Vincent Ferrer position, in other words, even if its wrong, the whole matter is doubtful and thus one could legitimately take either position without objective schism. 

Like we're not talking about whether individuals can be in good faith here, we're talking about the thing's objective status.

Two or more plausible papal claimants = You aren't in sin regardless of which opinion you have, maybe even if you think they're all  invalid.

One definite papal claimant (such as Pope St Pius X) = its schismatic to knowingly refuse submission.  If you'd just never heard of St Pius X that would be one thing, but if you realized the whole Church accepted him as Pope and refused to accept it you were schismatic.  Basically this is like the Assumption of Mary issue, a priest in a remote missionary territory might just not know about the ex cathedra pronouncement, but you can't reject it.

Post Vatican II Popes = ?

On the one hand, there are no plausible alternate claims (even if you buy Siri theory, Siri never tried to exercise any papal authority, if he had, that might create more of a point).  The *Entire* world accepted John XXIII, and early Paul VI, as Pope.  After that, some Traditionalists have dissented, ,but until Francis at least, there has been no dissent within the juridical hierarchy.  *I don't see* how Lefebvre and Williamson having *doubts* (though even still, they did/do accept the Popes) really undermines the universal moral acceptance of the entire Church.  Perhaps Lefebvre was incorrect to speculate along these lines, its possible.

*on the other hand* it seems wrong to say that someone who just can't square the idea that a Pope that worships Pachamama is a real pope, is in the same boat as somebody who would reject Leo XIII or Pius X just because they don't like authority... or something.

Also Lefebvre seemed to think it was acceptable to question, and while he could be wrong, he deservedly gets a lot of weight among traditionalists.

So yeah, a priest who simply hasn't heard of St Pius X would certainly be in the clear.  And if there's some priest deep in the jungle who's somehow still praying una cuм pius XII because he's somehow never heard of *any* of the conciliar claimants, he'd be in the clear too. 

Whether it follows that people who don't think Francis or JPII or whoever are Popes, and instead either think nobody is pope or that there's some hidden or secret pope, are covered, well I can't figure that one out, so I leave the judgment to God who is a good and righteous judge, and I trust that he'll handle the situation rightly.