I have read that that any part of the skin above the dermis can be regenerated, does that mean that the melanocyte layer can be regenerated too? if that is the case, why does disease like vitiligo happen?
Yes, they do...although an area of skin that has been damaged may heal with either hyper- or hypopigmentation. The melanocytes are at the base of the epidermis and transfer melanin pigment to adjacent keratinocytes. Damage to the superficial layer of the skin (epidermis) will heal without scarring. Damage to deeper levels will cause scarring. Adjacent melanocytes can migrate in during healing but depending on the case, the scar may be darker or lighter than usual.
As for vitiligo, the cause and pathogenesis is not well understood. Some believe it is an autoimmune disease but other factors such as genetics and lack of melanocyte growth factors may contribute to it.