Have you checked for internal parasites?
Most of the time, WC Uros carry a variety of them.
A vet can do it for you, provided you bring a FRESH fecal sample in a sterile jar to him/her.
Pinworm eggs are easy to see with a microscope, but other more harmful parasites such as amoebas or coccidia may infest your animal.
Meanwhile, keep humidity up- don't turn ithe tank to a swamp, but spray at least every evening heavily his cage. As Elizabeth said, keep temps around 80°F during the day and switch off any heating aggregate at night.
What your gecko needs is insects. Vitamins combine with proteins and other nutrients, if you give vitamins alone, that won't help much.