Regardless of the form of the hexahedron, for simply-connected spaces, the Euler Characteristic is 2, giving the formula 2 = V - E + F, where V is the number of vertices, E is the number of edges and F is the number of faces. In this case we have 2 = V - 12 + 6, or
V = 2 + 12 - 6 = 8.