The vat is the container in which they reduce the plants (or fabric) into the fibers from which the paper is formed. A su is a screen on a frame that is used to make the paper. They pour the suspended fibers from the vat onto the screen, and the water drains out, leaving a layer of matted fibers. This layer of fiber is pressed and dried to form paper.
If you gave us more context, we might know why she is dropping the paper back into the vat.
Adventrue: this is what we're looking for in terms of context:
From The Time Traveler's Wife, Audrey Niffenegger:
Clare: It's Thursday afternoon and I'm in the studio making pale yellow kozo paper. Henry's been gone for almost twenty-four hours now, and as usual I'm torn between thinking obsessively about when and where he might be and being pissed at him for not being here and worrying about when he'll be back. It's not helping my concntration and I'm ruining a lot of sheets; I plop them off the su and back into the vat.
And this is what I found: Tools Used To Make Washi - Japanese handmade paper Su… flexible screen, usually bamboo but sometimes miscanthus, that acts as a sieve or strainer upon which the paper sheet is formed.