Waht is a Vorpanzer?

its the term used when extra armour has been added. I think its more added on armour rather than extra armour installed at the design stage. I am not sure if its a term the Germans used.

Pretty sure that on the Pz III L, the only additional armor is the "spaced" armor on the mantlet and the front hull around the driver's compartment. Basically it works on the same principal as the schurzen, by breaking up the round before it strikes the main armor.

Vorpanzer refers to spaced armor. In the case of the Pz III, the idea was simple enough...you added an additional 5-10mm plate with a space/gap between it and the main armor of the hull superstructure front (the driver's plate) or the turret mantlet/face plate. Rather than just increase the thickness of the plates in question (and adding substantial weight in the process) you could achieve similar levels of protection with thinner plates and disrupt the ballistic characteristics of the impacting round and decrease the chances of a full penetration hit. That was the theory at least...in reality it didn't really do all that much and was only applied as a stop-gap up-armoring measure for the Pz III design until it was replaced as the MBT by the long-gun Pz IV and the Panther.