Many agencies require the wearing of ballistic vests while on duty. Why so many don’t is a separate topic. When using take home vehicles, carry a ballistic vest inside your car along with your weapon, and handcuffs.
You never know when you may have to respond to an emergency. All to often, officers do not think to do this as most agencies require you to only be armed off duty. If you’re in your car and identified as an officer on or off duty or going to & from training, Keep a vest with you.