Poor thing.
If she's not responding to the vitamins at all, I think you should still continue to administer them, but I do believe its time we start to consider some other problems instead. I see this limber neck activity going on, as well as her lack of interest in things. Would there be any chance she ate any pieces of metal, or ingested something moldy, or toxic? How does her feces look?
I'm not one to prescribe/mention the use of antibiotics unless I'm sure they will be useful, but that might be something I would try with her. Do you have any antibiotics on hand, if so what, and how many? If you don't have any, you can easily get some online in fish or bird form. For such a young bird I might recommend Amoxicillin. Would you be willing to purchase some, if so, here's a link to Amoxicillin.
https://www.chewy.com/fish-aid-antibiotics-amoxicillin/dp/185201#:~:text=You can help your finned,negative bacteria plaguing your fish.