He seduces Rosa, his brother's fiancee who hopes to reform him but can't resist his determined advances, which he makes in part to show her it's not so easy to be good in a bad world.
With the money in hand (leaving several dead guards behind), the other heisters turn on the ill-tempered domineering Vincent, whose arrogance and inability to understand other people's points of view and anticipate their reactions, prevent him from spotting the betrayal and heading it off.
He's the only surviving member of the gang the police know about, due to his connection with the gas station, so they get away clean, to rob another day, splitting the money evenly between them.
Johnny, knowing now what happened to Rosa, takes him at gunpoint to a local dump (the film opens with a flash forward of them driving there)--but can't bring himself to kill Vincent.
Aside from Lawrence Tierney's finely tuned, menacing performance, and Lisa Golm's Oscar-worthy deathbed scene, the acting was sub par.