It was once worn by Jarek while he was still an apprentice during his early days within the Black Dragon organization.
Where to find it?
The vest is in Botan Jungle. Once you are near Argus' Temple you will have to fight members of The Black Dragon Clan they are coming out of little boxes made out of cement. Once you defeat all enemies you are allowed in the boxes Jarek's Vest is in the first one to the left.