First thing you should do is take your cat to the vet, because lethargy and dull fur can be signs of health problems, not just a vitamin deficiency. It could be anything: from worms to thyroid issues. After the examination, the vet will let you know whether your cat actually needs supplements or if that's not the issue at all.
If your cat is healthy, quality premium food usually contains everything necessary. The vitamins are already added in the right amounts. But there are situations where extra support doesn't hurt - for example, during shedding season, when stressed, or with age-related changes. In those cases, it makes sense to give vitamin treats or supplements, but only the ones your vet recommends. Everything else at the pet store - yeah, that's often just marketing.
Also pay attention to your cat's living conditions: maybe she doesn't get enough movement, or the dry air from heating in your house is affecting her coat. Sometimes the problem is simpler than it seems.