Theta with Vandersteen... what cables?

I'd DEFINITELY go with something more extended, even more foreward/prominent in the trebble for your setup! Everything in that system, including the cardas, is a bit recessed in the trebble!
If it were me, I'd be looking at a minimum of Kimber's stufff, Harmonic Silver ref's, Acoustic Zen Matrix Ref II's even, or other hotter(trebble) cables.
Vandies are laid back on top, if a little veiled, and the Theta is also warm sounding and forgiving a bit, and cardas too (although they're solid, dynamic, big sounding cables). Good luck

I also use kimber select 1030 and KCAG with my dreadnaught with good results

Two cables I would recommend, Cardas Golden Refrence XLR, or the one I use, Audioquest Cheetah XLR (or Sky-a touch better.)
I have been using Theta with Vandies for sometime now. Tried a lot of cables. Very revealing and neutral. Cardas does add a small touch of warmth but a lot of Theta dealers recommend them. Much better than Cross.

I've heard Kimber Cable's on Vandersteen and Theta at my local dealer and it sounded good. He also uses Transparent speaker cable on them. Either one will be a little more open on top versus the Cardas.