Hm... I think Vortex wanted to leave Borknagar so he could devote himself to Dimmu. The only thing that keeps me REMOTELY interested in new Dimmu is Vortex, as I enjoy his bass playing and his vocals give the songs a sweet sound (at least when he's doing them). I'm a HUGE fan of old Dimmu (right after Enthrone Darkness came out I just lost interest), and the new stuff just isn't doing it for me. But as we are on the topic of Vortex, if he for some reason did go back to Borknagar, that would just be... oh lord it would be fantastic.
By the way- I wouldn't trade Vintersorg's vocals with Borknagar for anything!
Now I'm just sitting here thinking of the greatness that could be acheived if Vintersorg and Vortex both sang on the same cd. Oh what a glorious day that would be!!!!!!!!