It’s Z3D only, although I honestly think it’s working quite well with Thief (and also System Shock 2, which is similar engine wise). These games where made long before DirectX 9 and their DX9 ports work very different than almost all other DX9 games. I always wanted to look further into that, but so far there just never was time for it.
These games do all the geometry transform on the CPU, so no way for vorpX to inject G3D. Direct 3D (also true for the D39 versions of these games) is only used to draw the already transformed vertices to the screen. Hence they are Z3D only.
I have a few ideas for an additional very-old-games-renderpath that could handle such cases, but that’s one of those things that may or may not happen down the road, so don’t hold your breath. Would be nice to have, but is not a priority.