Season 2 of Call of Duty: Modern Warfare 3 is live, adding the new Ninja vest perk for customized loadouts. This new perk is geared towards stealth and the resupply of lethal throwing equipment, but first you’ll need to unlock it.
What is the Ninja vest
The Ninja vest is a new perk that can be equipped in your custom loadout to eliminate footstep sounds, provide immunity to movement reduction effects, and provide additional supply of lethal throwing equipment.
Activision says this perk does not stack with the stealth effects of the Covert Sneakers boot perk. However, this means if you use the Ninja vest for stealth, you’re free to select a different type of sneaker, such as the Lightweight Boots or Tactical Pads for movement buffs.
Source: Gamespot How To Unlock The Ninja Vest In Call Of Duty: Modern Warfare 3