Most guys who get VangComp treatment opt OUT of the perforating. It's supposed to reduce recoil, but it increases noise and cleaning effort immensely.
To be quite honest - I wouldn't bother getting it Vang'd on a 12" norinco barrel. The barrel is really thin, and not the highest quality.
On a remington or dlask barrel, sure... but I wouldn't waste it on a norinco. I'm no norinco snob - i own or have owned over 10 HP9's... but it's like pimping an SKS... doesn't increase the value and you'll take a huuuge loss when selling.
Buy an 870 Police and have it VangComped... that makes sense. Not a Norinco.