I don't know anyone has thought of it yet but with the new developments I can't help but think that Gantz created the vampires. Aren't the "vampires" just humans with the nanomachines inside them?
Either way, Gantz has not sent the Gantzers out to directly fight any of the big shots of the vampires. Their encounters with each other have been outside of Gantz missions and I believe that they were pitted against each other to weed out the weak.
With the previews of the Gantzers fighting alongside Gantzers from other places I think that the alien threat is more emminent now than ever so Gantz is calling in all their resources, including the vampires they made, to combat it.
I think Kurono and Izumi will probably be revived sometime later after Katou gets some development in pwning skills. Then the humans & vampires will team up with Gantzers of other nations to kill or be killed by aliens bigger than the Oni Alien.