It isn't your fault, even with the damage of the egg, there's no telling what actually caused this, if it passed. The danger from the break was bacteria, but this late in incubation it shouldn't have mattered that much. And even if it did, we all make mistakes in hatching, it isn't easy and we just have to learn from them
If there are veins, it will depend how thinned out they are. My prediction is that they will be very shrunk down, and dry. Moisten the membrane, so you can tell them all clearly.
I think you'll be okay, just ensure there are no veins in the way where you want to tear (just a small tear, just a little hole) the membrane. If you accidently hit one and there's bleeding, just hold a dry paper towel to it until it stops. I don't think there will be much blood left at this stage