Friends that dress up in vests to social gatherings and events that one would not wear a vest to.
"Why are you guys wearing vests at a (football game, roller rink, game of paintball, etc.)?"
(Together) "Were vest friends."
A person you don't necessarily know, but the person is in fact wearing an almost identical vest as you do, and therefore you feel obligated to initiate a dialog.
Vest guy 1: Hey man! Nice vest!
Vest guy 2: Hey! Nice vest man!
Two months later at Waffle House:
Vest guy 1: Hey man! I recognize that vest!
Vest guy 2: Howdy! Yeah! We're like vest friends now!