Go to any clothes or gun shop and purchase a heavy utility vest/heavy combat outfit and/or a bullet proof helmet then save them as an outfit. In the lobby the host can change "heist clothing" to "player saved outfits" instead of "from selection". This will provide extra armor in heists, but limit your ability to sprint but it's a price worth paying.
Scarf Trick -
When wearing a player saved heavy combat vest on a heist, go to style, accessories, and then equip a scarf. This will remove the vest, but keep the damage resistance it provides, it will also allow you to sprint. Use this at your own risk as being able to sprint is not necessarily an advantage. Scarves can be bought from clothes shops at the counter.
Scarf Trick -
When wearing a player saved heavy combat vest on a heist, go to style, accessories, and then equip a scarf. This will remove the vest, but keep the damage resistance it provides, it will also allow you to sprint. Use this at your own risk as being able to sprint is not necessarily an advantage. Scarves can be bought from clothes shops at the counter.