I'm caucasian and have vitiligo. Should I use broad spectrum sunscreen (uva/uvb) or just the uva? Will uvb rays help?

Both. The more tan you are the contrast there would be with the white slin.
Vitiligo. Narrowband ultraviolet b (uvb) therapy. Narrowband uvb, a special form of uvb light that uses a more specific wavelength of ultraviolet b, is an alternative to puva. This type of therapy can be administered like puva and given up to three times a week. However, no pre-application of psoralen is required, which simplifies the treatment process for vitiligo.