Go to any PD or Delve or anywhere but a siege camp and Vitrified Souls will drop easily. Go to a Siege Camp and it becomes very much hit and miss - sometimes it's a flood and sometimes it's a drought. I have a theory based on my own observations.

My Theory:

1) Not every mob killed is lootable. The % of mobs at siege camps that can be lootable is hard to judge (given what I am about to lay out) but on my observations and typical behaviour I would estimate it is somewhere between a 25% - 50% chance that a killed mob will be lootable.
2) If YOU or a member of YOUR group get the killing blow, you will get loot privileges - if it satisfies 1. Now what consitutes the killing blow? Strictly speaking the last tick/hit that takes the mobs last health. The actual could be anywhere from the last 1 - 6 players to damage the mob which would be consistent with how loot privilege seems to work with overland mobs. Bosses seem to take a different approach being the highest 12 damage dealers, though this has altered over the years to include tanks and taunt.
3) If YOU heal someone not in your group, who then satisfies both 1 & 2 you will get loot privileges - again this would be consistent with how loot privilege seems to work with overland mobs.

I have consistently found that if I am throwing out an AoE HoT (due to the fire and forget as opposed to a standard one off heal) that actually heals rather than overheals then Vitrified Souls dropping from Siege Camps are a lot less hit and miss and the flood/drought becomes a steady stream.