A pot of vinegar can be made by pouring a jug of vinegar into an empty pot. During the Rag and Bone Man I quest various bones that are dropped from creatures around Gielinor can be added to the pot (one bone per pot, 8 pots total) 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 (you don't have to bring them all together in the quest though).
