There's actually quite a few canon units and designs for Vong ships, far more than for the Chiss or Empire of the Hand. As for Dovin Basals Corey and Code figured out years ago how they'd like to handle those in EAW (assuming the game would behave itself).
It was more designing and creating the organic texture that would be the problem, I would assume. Mechanical is both easier in general to make look good, easier to code for movement, and there's already similar meshes/code/etc from the rest of the factions that can be extrapolated from. I doubt tentacle-worms (qworlth skaal) would be easy to code, for example.
(assuming the game would behave itself).
Also, the more important part.
I have faith in the TR team's coding ability. Not so much in the game