A vat is a decorative item that generates in certain dungeon models. Many vats in dungeons appear to have bits of green acid dribbling down the sides.
Vats are used in various builds to suggest an industrial tone.
How to Obtain
Vats can be mined in dungeons or crafted.
Originally, the only way to obtain vats was to mine them from dungeons. This was fixed in a [time frame] update, where they could also be crafted.
Vats appear to have a port at their bottom left block, which suggests they will perhaps someday have a use when linked to steam power.