To spawn Lord Vyemm, its an event of 4 PH, killed in order. The next one spawns upon the death of the previous one.
1) a vile defender 2) a vicious scholar 3) a vicious warrior 4) a vicious leader
Upon the death of the vicious leader, Lord Vyemm spawns. Lord Vyemm paths around in a circle on the lower level at the entrance 4-way by illustrated in the red circle on the map image.