Green’s Dictionary of Slang

vest n.

1. (US) a bullet-proof vest.

1933		Ersine Und. and Prison Sl.
1988		Ice-T ‘Personal’ 🎵 I hope those punk reporters wear vests.

2. (US Und.) watches and/or jewellery worn on the waistcoat.

1952

C. Hamilton Men of the Und. 326: Vest, Watches and jewelry worn on the vest [UK waistcoat].

SE in slang uses

In phrases

lose one’s vest (v.)

to lose one’s temper.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.

In exclamations

pull down your vest

(US campus) stop (doing something)!

1911

E.B. Morris Sophomore 7: All. Come off. Chuck it. Pull down your vest.