It's worth noting that, in clothing terms, the writeups above give the
American definition of 'vest'. This appears to be some kind of sporting jacket with pockets, commonly worn by
backwoods types.
In the UK, a vest is a close-fitting, sleeveless, buttonless garment which slips on underneath a shirt. Vests are almost always white. A vest's purpose is three-fold; firstly, it provides warmth, secondly, it helps to absorb perspiration, and thirdly, it prevents a man's nipples from showing through thin cotton shirts. According to... (puts finger in ear)... informed sources... the US equivalent of a British vest is called a 'singlet' or an 'undershirt'.
The stereotypical outfit of a drunken bum (such as Gregor Fisher's popular 'Rab C. Nesbitt' character) consists of a ripped jacket and a stained string vest (a string vest being similar to chainmail, but made of string). For some reason, vests are associated with the North of England.
Sports vests are similar, but thicker, and usually have logos.