In the last brilliant chapter, Yhwach the leader of all Quincies stated Uryu was the last living Quincy of this world. Then what are the Vandenreich? There Quincies that's for sure, they have crosses, abilities, Yhwach stated that Quilige was a pure blood Quincy.
There are many assumptions but I think these Vandenreich are the souls of Quincies, Vandenreich state could be the Quincy equivalent of the Soul Society, Yhwach perhaps handpicked the souls of deceased Quincies and choose the strongest for his army.
What do you think?