I don't see why forging the X-blade would be an issue as long as you knew how to extract the darkness from someones heart. Because all-in-all it wouldn't have really mattered whether Ven lived or not (Vanitas was alive and kicking even while Ventus HoPL was in his comatose), seeing as Master Xehanort had Aqua as his "spare." So as long as you had a spare and a Heart of Pure Darkness, wouldn't it be quite possible to create the X-Keyblade?
that depends if vanitas dies with ven, even seperate there connected the reports even say vanitas feels some of waht ven does.
and if he did survive they could have easily used a PoH.