pvb[x].u1 = 0;pvb[x].v1 = 1

Here you have to set the UV mapping. 0/0 is the top left pixel of the skin, 1/1 the bottom right pixel of the skin. With the current setup you always use the full skin.

So you might want to choose one side of the cube to be at 0.1/0.1..0.4/0.4, the next side 0.6/0.1..0.9/0.4 the third side 0.1/0.6..0.4/0.9

You may need to use more vertices to get a proper UV map.