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 .... 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.
For example st. Bernard of Clairvaux and Thomas Aquinas could be called material heretics because they denied the immaculate conception of our lady.
Do only formal heretics vacate the papacy, or do material heretics vacate it aswell?