ORIGINAL: stubblejumper
It is quite common for gun vices to cause a different point of impact.A gun vice that prevents the gun from recoiling can also result in a cracked stock.
A cracked stockcan be a problem. I would use a vicefor adjusting the sights to reduce the time and ammo for sighting in, but I never SHOOT from a vice. It is also true that having the butt of the stock against anything other than your shoulder will most likely affect the POI.