Instead of rectangles, then, the faces are all parallelograms. So if we consider the face containing , and , the fourth vertex of this parallelogram - , say - is such that:
Then deal with the 2 faces containing , and ; and , and , respectively, in the same way to get, say, vertices and .
Finally, we shall need the eighth vertex, , say, whose position vector is the sum of all three vectors . This vertex will be diagonally opposite .
Can you complete it now?