Osiris is a guardian not a vex I don't think he will be evil in the vex dlc however I think you will have to choose between Osiris and the Speaker since Osiris was baned from the city and if you would choose Osiris there will be a new social area on mercury like the lighthouse when you finish trials 9-0 and the raid in this dlc would be that you destroy the vex for good. And Osiris has nothing to do with the vex