I haven’t had the opportunity of raising peafowl, but I also have a bird with wry neck.
I will agree with the other user that giving half the dose may suffice as well. I will also add that sometimes birds vary with how long it takes them to perk back up, but noting that it usually doesn’t take long. I’ve had my silkie chicken in the house with wry neck since February and still she is in the house because she won’t keep her head up—she eats and preens fine on her own, she just puts her head down often and seems stuck.
Since the peachick was born with wry neck, is it possible that it’s more than just a vitamin deficiency, such as it potentially developed improperly? Or developed with weaker neck muscles? I’m no expert by any means, but I like to consider all possibilities just in case.
All I can recommend from my experience is to encourage them any chance you can, to use their neck and build up their strength. Keep doing what you’re doing and have a little patience. Unless the chick is refusing food or is unable to eat, I don’t see why not to keep trying. Best of luck, though!