The phrase "big vat" refers to a large container or tank used for holding or storing a large amount of something, like liquid or a substance. It is often used to describe a large quantity or volume of something. Full definition
Seems, if memory serves, that just about everything being dished up by the nice (cranky) laddies dressed in all white uniforms (not unlike nurses) was obviously from a can or box and simply heated in big vats or off a grill. (thelunchtray.com)
A few months ago we could have equipped the expedition that rowed to somewhere near where the Magnetic North Pole used to be about twenty years ago with a superdooper big vat of Ambre Solaire for the Polies. (judithcurry.com)
By trial and error, they came up with a process in which the spores are fermented in big vats, with glucose as a food source and various other nutrients. (scientificamerican.com)