which first aid treatment is appropriate if the...

The vein cut is an emergent condition which required immediate first aid to stop the bleeding as much as it's possible

What are the first aids that should be done?

Determine the severity of the cut if it's heavy and serious or small
Maintaine the Airway, Breathing and Cercualation
Help the injured person to keep calm
Call for medical help
If the cut is large and heavy, help him to lie down
If the wound is on arm or leg, raise the limb above the heart
Remove any debris around the wound
Wash the small wounds by clean water
Apply firm pressure to the wound by bandage of folded cloth for 10 minutes and then re-evaluate the wound
If the wound is small and bleeding has stopped, tape a clean bandage over the cut
f the bleeding is heavy and has'nt stop yet, continue applying pressure until medical care arrives

What are thing you shouldn't do?

Don’t remove an object if it’s embedded in the body
Don’t attempt to clean a large wound