Can a vortex viper fit in the MOS slot?

Yes sir Ken it certainly will as pictured : google.com/search?q=vortex+viper+red+dot+g34&source=lnms&tbm=isch&sa=X&ved=0ahUKEwip55mtj_DbAhVCR6wKHQu2ALcQ_AUIDCgD&biw=1280&bih=618#imgrc=w2dyyHijsj96MM: