You need more vertices, not less. A cube has 8 corners and you can join these up using triangles to make all the faces. What you can't do with only 8 vertices is get the uvs right on all faces at the same time. Some will be jumbled up and I thought this was probably your problem.
To get the uvs right, you need 4 vertices per cube face, i.e. 24 vertices. Each corner then has 3 vertices at exactly the same location, but now they have the possibility of having distinct uv coordinates.