Q: Suppose I have a couple of Myrs (say ten) and two Disciples of the Vault. My opponent plays a Wrath of God or Pyroclasm. Does it mean that my opponent will be dead due to my two Disciples?
A: Yes, it does. When dealing with abilities that trigger off permanents leaving play, the game looks back in time to right before the permanents left play to see if anything triggers. All the Myrs left the play at the same time, and right before this, the Disciples were in play. Each destroyed Myr would trigger each Disciple of the Vault, and your opponent would lose 20 life.