It's very likely that Viper will have some sorta stance thing going on, atleast in the moveset and not actual stance buttons, you can go to 0:20 in the official reveal trailer and see the first idle animation, and then if you go to 0:37-0:38 you can see the idle animation change when the swords were connected together. I could also be wrong and this will be a duration sort of thing, like with Reaper, but maybe Viper will be the first job to have two different idle animations depending on your stance(?).