Vigilante missions involve taking on the role of a law enforcer, hunting down criminals across Vice City. To start a Vigilante mission, enter a police vehicle (Police Car, Enforcer, Rhino, Hunter) and press the designated button (usually '2' on PC). You'll be tasked with eliminating a certain number of criminals within a time limit. Completing levels of Vigilante missions will increase your maximum armor capacity, making them incredibly valuable.
The difficulty of Vigilante missions increases with each level, with more criminals and tougher opposition. It's recommended to use a durable vehicle like the Enforcer or even the Rhino (if unlocked) for higher levels. Stock up on ammunition and health before attempting these missions. Reaching level 12 will grant you the maximum armor bonus, making Tommy significantly more resilient.