Welcome to AquariumAdvice.com
The star sounds like it's not getting enough nutrient. The vitamin you are adding is not really going to help much (if at all). The small size of the tank really cannot keep up with the stars natural food source. Primarily film algae. It is not really recommended to keep a star in much less than a 100 gal
loaded tank for that reason. They will do fine for about 6-18 months and then simpley diminish. They actually begin to shrink in size.
My best recommendation is to try suppliment the available foods with sinking marine pellets and nori in hopes it may stumble across them and eat them. Chances are it won't though, they rarely do. The only other suggestion is find it a new home that is better equiped to support it until you can upgrade and the newer larger tank has matured enough to sustain it better.