As far as the “completely vitrified” question goes, remember some dinnerware is bisque fired to the highest cone while being supported by a sand mold to prevent collapse. All glaze and decorations are lower fired step by step. That kind of ware is probably completely vitrified.
Absent X-ray diffraction studies to determine a estimate of remaining crystalline particles, fracture patterns could hint at vitrification level. A completely glassy object should exhibit conchoidal (curved) fracture; a less glassy object should exhibit a rougher fracture surface.
Master’s degree topic for someone, not me!
Encouraging everyone to do fracture studies, especially with work you don’t like.
I would not discount the possibility of a clay body retaining (or generating) enough crystal character to prevent collapse while still allowing enough melting to fill pores. Clearly, the shape of the object is important but glasses can be extremely viscous even at elevated temperature.