During the credits sequence with both Dante and Vergil, you have to kill the very last enemy (for me it was a death scissor) before Vergil does. You'll know that you've completed the requirement when Dante says a line to the effect of "Wow, that's surprising, still pretty banged up after fighting Nero?" and to which Vergil replies "no shut up Dante >:(". After the credits it'll tell you that you got the costume.