Theta with Vandersteen... what cables?

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

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