I love Dimmu Borgir (and i'm not a mouth breeding gutter ape) and i think that if you want to hear more of Vortex, just listen to some Borknagar.
I like Shagraths vocals and i like the way that Vortex blends his in as well, do i wish that Vortex did more? hell yeah i do but i also reckon that too much and it might start sounding too much like Borknagar.
take Allehelgens Dod I Helveds Rike, the one small bit of clean vocals in that song always sends a chill down my spine when i hear it and the fact that it is only a small part helps the effect because it's such stark contrast to Shagraths vocals.
Vortex himself admits that he only puts in vocals where he thinks it would fit but he doesn't want too much of it in there.