Open a Vaal Vessel in a corrupted zone.
A Vaal Vessel is a loot container found near the boss in all vaal side areas and in the Vaal Temple Map. The vaal side areas versions can only be opened after the boss has been defeated. The vaal side area version always contains Vaal Skill Gems or Vaal Fragments. Sometimes it contains corrupted gear.
A unique version, Atziri’s Vessel, can be found in the Vaults of Atziri map. Opening it will complete the map objective and it will drop multiple Sacrifice Fragments or Vaal Skill Gems.
The Vaal Temple Map is more behave as a strongbox.
The drop distribution from Vaal Vessels in vaal side areas were stated by Chris Wilson in 2014:
This data does not reflect subsequent changes such as the introduction of the Vanity divination card in Version 3.7.0 into its base loot table, as well as the vaal side area rework in Version 3.10.0, so this data may outdated.
Depending on the modifiers of the vaal side area (in spawn weight table known as “secret_area”), the Vaal Vessel may also contain additional corrupted or Vaal-themed items.