This was the hero vest worn by the Merc played by Nigel Vonas - Part of Toomb's 5-man team. It's seen very clearly throughout the scenes on board the skiff, the underground tram and the Crematoria Slam sequences. Probably most prominently seen after Riddick causes the death of the Merc on the moving tram, Toombs taps the chest of the merc wearing this vest and shouts 'Four way split!'
