Well this took a while... anyway I was trying to recreate one of my Sims 4 sims in Sims 2 and wanted all the skin details to match up perfectly so I converted the vitiligo textures to Sims 2 in an overlay box.

There are 71 separate swatches, split between arms, legs, upper torso, lower torso, and face. Face textures do work with other overlays. The preview shows multiple different skins and skintones so you can see what they look like. (The one guy is backwards because otherwise he'd be sideways). For the vitiligo textures themselves I used the Tattooer to get the general shape of the textures over and then used a custom tone of Dreadpirate's Party for One skinblend to make sure the details weren't lost on the face. The overlays work on both male and female, ages toddler through elder. There are a LOT of choices (between 8 and 21 for each body part) so I recommend making a note of what you picked in case you ever want to remove them. The box is a mannequin head (using the same texture as the birthmark overlay box because it fit the theme) and is located in misc > dressers.