Do you know what breed she is and how old? I would be concerned that it might be Marek's Disease, They are often bright eyed and eat well with it but it can affect one or both legs or wings or neck initially and unfortunately often progresses to organ failure as tumours develop internally. She may be small because she has been bullied and kept from the feeder at the farm or she could be small as a result of Marek's... they often have stunted growth.. If you have any other chickens I would be very careful about quarantine.
You are doing the right thing by giving her vitamins and good quality treats.... the important thing is to keep her happy and stress free and well fed.
I've had chickens survive Marek's so it's not all doom and gloom if that is it.
Good luck with her.