A jug of vinegar can be bought from Fortunato in Draynor Village for 1 coin after starting the Rag and Bone Man quest. When decanted into an empty pot it becomes a pot of vinegar and an empty jug. Bones from various creatures around RuneScape can then be added to the pot (one bone per pot) and boiled over the fire outside the Odd Old Man's house. This will clean the bones and they can then be given to the old man to complete the quest.
It is also used in the Rag and Bone Man and Fur 'n' Seek wish list miniquests. After the miniquests, the vinegar can no longer be bought from Fortunato.