How do you take off body armor in GTA Online?
You can just hit the Select button on the PS3 or back on the Xbox 360 to access Inventory. There you can turn off the Armor option
Go to your closet in your house. There you can deselect body armor. Scroll down to another option in the shirt or pants section and select something else.
Or, on xbox, press back, go to inventory and press body armor and change it to none.