Body Armor is crucial for survival in Vice City. It acts as a secondary health bar, absorbing damage before Tommy's health is affected. Armor can be purchased from Ammu-Nation, found as pickups in the environment, or obtained by completing Vigilante missions (which increase your maximum armor capacity). Always try to have full armor before starting a mission.
There are different levels of armor, but the most common is the standard Body Armor. The maximum armor capacity can be increased to 150 by completing Vigilante level 12. This upgrade is permanent and makes a significant difference in your survivability during intense firefights. Never underestimate the value of a full armor bar.