George Patterson popped their head over the parapet saw what was going
on and said
soup wrote:

Is an undershirt and a vest not the same thing ?

Not in the U.S.. Here, an undershirt is a thin pullover shirt worn
underneath the shirt. It's also called a "T-shirt." It's usually made
of cotton or a blend of cotton and synthetic material.

Ah a "sloppy joe" (blast from the past name) if it is worn under a
shirt it can be called a vest depends on how it is worn rather
than what it is.
cf "string" vests for older chaps, Ts for younger chaps

Remember once wearing a T under a shirt, the shirt
was one colour the T had a slogan on it, some people
spent an inordinate amount of time trying to read the slogan.

--
yours S

Nihil curo de ista tua stulta superstitione