This is an piece of armor. Drag it to it's corresponding slot in the armor window and it will provide additional protection
The Invisible Vest was part of a special armor set. Each item in the set gave 50 protection on all available attributes. While wearing the complete set the player would be completely invisible in the game.
The vest could only be obtained if an admin spawned it into the game using the 'inv.giveplayer “NAME” “Invisible Vest” 1' console command.