I don't know if I'd jump to culling her, this is (unfortunately) pretty common in breeds with vaults (Silkies, Polish, ect). Vitamin E and selenium are the usual recommended treatments. Nutri-Drench is an excellent energy boost, and that would really help the little thing right now (it may also already have the E and selenium in it, not sure).
One word of caution, if you decide to work with her, this might be an issue for her for the rest of her days. One of my Silkie roos suffered an injury to his vault as a youngster. He took a peck on the head when he was introduced to my flock. He would "head bang", walk in circles, run into things and tuck his head between his legs and do somersaults. It was pretty bad. I treated with vitamin E capsules and selenium and after a couple of months, he was much better. He lived a long time, I had him about 8 years, but he always had a head wobble. Most of the time it was slight, but other times (when he was stressed) it was pretty severe. When it would get bad, I'd go ahead and treat him again to get it back under control.
Good luck with your poor little duckling!