It's useful for normal plaque psoriasis, but I'm currently recovering from an erythrodermic psoriasis episode and UV light is not recommended for that. Psoriasis can be a tricky condition, as too much UV light (i.e. sunburn) can trigger it - UV therapy usually has to be carefully controlled.

Light therapy is very effective, but not because of vitamin D. UVB light has an immunosuppressant effect on the skin.