A hole. The most common type of blood vessel injury is when a hole is sliced or punctured in the wall of the vessel, and it then bleeds through the hole. Just like running over your garden hose with a mower, the fluid can spray pretty far if it's an artery, or leak out slowly if it's a vein. Blood vessels can also be bruised, torn, and stretched in ways that don't cause bleeding, but may stop flow.
Depends. There are arteries and veins. Injury to an artery can be serious depending on which one is injured, and could result in significant bleeding. Major vein injuries can alao be serious, while asmall vein injury could just cause a little bruising and little else.
Compex. It could mean disruption of the three layers with blood loss, partial injury with intimal damage, thrombosis or stoppage of flow, dilation of vessel with aneurysm or pseudoaneurysm formation. There other complex forms of vascular injury like av fistula, external compression, etc.
Blunt or penetrating. Depending on mechanism of injury blunt or penetrating which can end up with small intimal (inner wall) dissection, wall hematoma, contained rupture with psuedoaneurysm formation, transaction or thrombosis.