It is like a shirt, but sleeveless. There is a big square in the middle that is presumably for the armor part of the vest, and it takes up much of the "stomach" area. Both sides have two sets of small squares that are aligned on top of one another on either side of the vest.
Related to Bulletproof Vest Icon
Related icons are the icons with matching tags, as well as all military icons. All icons are in the same flat style. You can either download free PNG icons or buy SVG vectors