"vacuum-formed" may to refer the process to make the panel Whereas PP, ABS, HDPE, etc, are types of material injected into said vacuum formation.
The OEM panels defy repair, similar to non-painted plastics from other cars, motorcycles, boats, etc.
Thin and brittle seems like material, more than process.
PP is pretty much called PP, Polypropylene.
think take-out food containers, straws, medicine bottles, and other difficult-to-recycle plastics...
by design. PP is very resistant to some types of wear other plastics are plagued by. But that resistance has a cost. Difficult to repair and recover (recycle)
Among those poorly suited to resisted to UV light, are PP and LDPE. So, most white OEM interior panels are crumbling due to UV exposure. A sad truth in a busy changing world.