You raise an important question: What was a labrum in Roman architecture? While Roman bathhouses had labra, the core sense of labrum is a basin. One could use a labrum for stomping grapes, for instance, in which "vat" is probably a decent English gloss. But in a bathhouse a labrum was a basin usually in the cal(i)darium, the hot room, and was used for splashing your face & neck with cool water. They were often round. Here's a link: http://www.vroma.org/~bmcmanus/baths.html For a deeper dive: Garrett G. Fagan, Bathing in Public in the Roman World (Michigan, 1999). The problem with "tub" is that for many English speakers a tub is a place where an individual bathes, which is not the sense of a labrum located 'in balneo' for Romans. The above link allows to click on a labrum photo. Here is another link: https://www.flickr.com/photos/11602696@N00/2899676532 You can see from the latter that a labrum was not deep. A person did not enter into a labrum in the balneum. Extant labra indicate that they were sometimes made of marble and sometimes even had a little fountain. A labrum for stomping grapes would probably be deeper. An additional observation: another sense of labrum was "lip," and often a labrum had a lip on which was written a dedication, as is the case with the very expensive labrum at the men's caldarium in the Forum baths at Pompeii. Source: Garrett, p. 252 # 68. You can see the inscription on the lip in the second photo above.