Vitiligo is treated in two halves, and which half you are in decides everything that follows. Active disease (new patches, existing ones spreading), is quietened first. Repigmenting skin while fresh patches keep appearing is effort spent against a moving target.
Stable disease is the one that responds to light. Narrowband UVB for widespread patches, excimer where there are only a few, usually alongside a topical. You are told at the first visit which category you are in, roughly how long a fair trial takes, and which of your patches are unlikely to respond to light at all.
That last part matters more than it sounds. Patches on the lips, the fingertips, and any patch whose hairs have gone white have lost the pigment reservoir that light works by recruiting. Being told so at the start is more useful than six months of finding out.