Doesn't the fact that John XXII denied the teaching that souls experienced the beatific vision before judgment day, prove the concept of a material papacy by the fact that he is included in the list of pontiffs ... John XXII did admittedly renounce his view near death, but since he was a heretic at the time of his election and during most of his pontificate, doesn't that prove that a heretic can be pope at least materially?

IOW, he was WRONG -- but not in an area where being wrong would involve heresy (at that time).  Further, as you noted, he changed his mind on the matter.

Even if a matter has not been clearly defined ... it is still true or false and a part of our faith, thus if a person denies it when it is undefined it is still heresy, it just is material instead of formal heresy ....

This is incorrect.  Material heresy is like what you wrote here:

its no different then when a person in ignorance denies a defined truth today, the difference is the later can be held slightly more culpable because his error stems in part from slothfulness in investigating the matter.

It is radically different, as you are here speaking of a situation where someone is simply unaware that something has been defined -- i.e., they are not aware that they MUST believe something.  J22 was wrong about something, but he was NOT wrong about something where the correct idea had already been authoritatively taught.

For example st. Bernard of Clairvaux and Thomas Aquinas could be called material heretics because they denied the immaculate conception of our lady.

No, they could not.  The matter was not declared as de fide until they had been dead for hundreds of years.

Do only formal heretics vacate the papacy, or do material heretics vacate it as well?

A man is not a heretic AT ALL unless his heresy is formal.

From John Daly:

Correctly, the material element involved in being a heretic is conscious dissent from the Catholic rule of faith, while the formal element is the perverse state of the will which this entails. The distinction thus made, a Catholic who inculpably advances a heretical proposition by inadvertence may perhaps be said to have advanced a material heresy; but he cannot be called a material heretic. He is not a heretic in any sense. A heretic is one who dissents altogether from the Catholic rule of faith, and he will be called a material heretic if he is invincibly ignorant of the authority of the Church which he rejects, and a formal heretic if the Church's authority has been sufficiently proposed to him, so that his dissent from it is culpable. (This is clearly explained by Cardinal Billot: De Ecclesia Christi, ed. 4, pp. 289-290)

From the Catholic Encyclopedia:

Both matter and form of heresy admit of degrees which find expression in the following technical formula of theology and canon law. Pertinacious adhesion to a doctrine contradictory to a point of faith clearly defined by the Church is heresy pure and simple, heresy in the first degree. But if the doctrine in question has not been expressly "defined" or is not clearly proposed as an article of faith in the ordinary, authorized teaching of the Church, an opinion opposed to it is styled sententia haeresi proxima, that is, an opinion approaching heresy. Next, a doctrinal proposition, without directly contradicting a received dogma, may yet involve logical consequences at variance with revealed truth. Such a proposition is not heretical, it is a propositio theologice erronea, that is, erroneous in theology. Further, the opposition to an article of faith may not be strictly demonstrable, but only reach a certain degree of probability. In that case the doctrine is termed  sententia de haeresi suspecta, haeresim sapiens; that is, an opinion suspected, or savouring, of heresy (see THEOLOGICAL CENSURES).

I was thinking about this example of John XXII tonight while walking the dog, how people use it against the sedevacantist position, and it got me steaming mad.

I don't know if Laurentinus is SSPX, but let me turn my basilisk glare towards the SSPX-ers who are here.  Why are the SSPX types so selectively knowledgeable?  Why do you pretend that there is ANY equivalence between a Pope who is mistaken on some theological point or other with a gang of Masons, Modernists and Jєωs who have hijacked the Vatican?  

When it suits you, when you want to justify your resistance of the "Pope," you'll bleat about Masonry and Modernism and so on.  But then when you want to carp against the sedevacantists, you'll call us crackpots for pointing out the exact same conspiracy!  Caminus has essentially done this with me, even though Abp. Lefebvre said just what I am saying, that this is Freemasonic usurpation.

A Pope who maintains an error as a private theologian or even when speaking publicly is one thing.  A series of Popes who serve the devil by trying to create a new religion on the ashes of the old, overhauling the Magisterium, the Mass, the Rite of Consecration, is another.  They are gutting the Church systematically.  

Why pretend that this is anything like what happened with John XXII, Liberius or Honorius?  This is not about an individual.  This is not about a weak Pope we're talking about, or one who was mistaken, or cowardly, or sinful -- this is a PLOT.  You who are in SSPX are aware of this, and many of you are more conspiratorially-minded than the sedes.  But to justify your half-in, half-out posture, you block what you know, that FREEMASONS AND THEIR STOOGES ARE NOT GOING TO ROLL OVER AND THEY DO NOT HAVE DIALOGUES.