It’s a model from MikuMikuDance (the japanese Source Filmmaker), so it’s going to have at least 50 different textures because the usual MMD model is colored by vertex with a few textures (same reason they rarely have arm IK rigging but 99% have feet IK rigging).
why are you talking about rigging? you dont always have IK legs and FK arms, it depends on the animation. most rigs lets you use IK AND FK on both arms and legs.
the reason legs often has IK is that it has contact with the ground, no other reason. if the character for some reason was walking on its hands, or was planning on leaning on a wall, you´d have IK on the arm and FK on the legs.
so about your model, vertex painting has nothing to do with texturing in this case. a material is often a single colour, and in your case you wont need more than 10ish materials for that character. if you currently have vertex paint on him, then wipe it and add solid colours instead.
if you wish to have any details, then use an image texture instead.
also, if you want to show rendered view of the character, please drop a sun light in the scene. really hard to see anything with such poor lighting.