Phototherapy is a standard treatment for vitiligo, which is why sun exposure can help. Patients with vitiligo actually have lower rates of skin cancer. This makes sense since the cells that could potentially give rise to melanoma are attacked by the immune system in vitiligo (depigmented skin lacks melanocytes). Still important to wear sunscreen to avoid burns and minimize the color contrast with normal skin, although some sun exposure may actually suppress the immune response and cause repigmentation.
That said, there are no great treatments. Topical steroids can work sometimes. Phototherapy as I mentioned. When the pigment comes back, it starts near the hair follicles (looks speckled).
Hopefully there will be better treatments available in the near future.