The veins are lightly attached to the inside surface of the egg, you can actually go between the veins and egg surface if you create a small hole then lightly keep upward force on the scissors keeping a "pocket" of air to separate the veins from the surface. Candle first to find a good spot to start.

That being said, cutting a vein will not hard the snake, it's happened many times with no issues. You'll just see some blood, be careful and sure your scissors are no where near the snake.