What is the relationship blood donation and vision?

None. Donating blood is just a simple procedure where a sterile needle is inserted in your arm and your blood will either be transfused into another person or be stored in a blood bag then deliver it in a hospital. This procedure has nothing to do with your vision.

Or, are you trying to say you'll get dizzy and your vision becomes blurry after blood donation? Is that it? If yes, get yourself enough rest and sleep. Drink plenty of nutritious juices and you'll be fine. If dizziness won't disappear, consult your doctor.