APV is a value of armour that it ignores. This is a fixed value and is sperate from damage.
E.g. if a person with 4pts Armour is shot with a round that does 2d6 damage and has 6pts of APV then 6 out of the 4 pts of armour is ignored (all in this case) and they take just 2d6 damage.NOT 2d6+2. The extra armour penetration is wasted.
The same weapon (2d6 +APV(6) ) against the person in 10pts of battle armour, 6 out of the 10 pts of armour is ignored leaving the person with 4pts armour which helps against the ramaining 2d6 damage.
Cthulhu rising has hit locations so damaged is dialed down slightly on weapons.
Although it doesn't specify it in any rules I would suggest that if a round has to go though two sets of armour, such as a person in body armour standing behind a wooden wall, the round keeps its remaining APV ONLY if it uses less than half penetrating the first armour, otherwise the APV drops to zero.
This would fromt he round sheading velocity and starting to tumble when it encounters enough resistance.