The two quickest ways to drain blood from the body is through the femoral arteries and the jugular veins.
The femoral arteries run through the inner thigh and around the back of the knee.
The jugular veins are located toward the back of the neck, behind the spine, and in front of the spine, right behind your throat.
The jugular is much easier to reach and bite, and it's a lot quicker. A vampire can bite anywhere in the body, but a bite in many other places would most likely begin the healing process before the body is completely drained of blood, unless there's constant sucking. Even if there is constant sucking, it would take much longer to get all the blood.
The jugular vein is there, and your body pumps a lot of blood into your brain to keep it going. It would also kill them quicker. That's why a lot of carnivorous जानवर also go for the neck when they attack a human.