The Russian Roulette stuff was from another screenplay about guys who go to Vegas and do it there. The scene was apparently so dramatically striking that Cimino imported it into the film despite it having not much to do with anything.
It is dramatically striking (at least when its initially done at the VC prison cage), though on repeated viewings when DeNiro goes back to Saigon to find Walken, it becomes more ridiculous than anything else. It’s highly unlikely that a heavily drug-addicted Walken would’ve lasted very long playing that “game” (a day, at the most). Of course, even closer scrutiny and you realize both of them are a decade too old to be playing Vietnam draftees.