I don't think it'd be particularly useful in general but I could give you a D3D8 build with vsync disabled for that specific case, to experiment with the game.

I already managed to do it for D3D8 by combining dgVoodoo with Special K, but am having a lot of trouble getting that combo to work with D3D7 and older. Those games simply refuse to launch. That was a bad assumption on my part. The problematic game is Thief 1 (It does more-or-less work with only dgVoodoo). Tried another game, Spider-man 2001, and the combo does work.

Please reconsider if you could make vsync disabling a general dgVoodoo feature. It's rather niche, yes, but it does have its uses. One such case is VRR displays. Usually disabling vsync and capping the fps gives a better, smoother experience.

If not, as an alternative, are you ok with trying to figure out why dgVoodoo+Special K is so problematic for pre-D3D8? I asked on the Special K discord server, but haven't received a response from the devs; probably are not interested.

EDIT:

I'd have to test more to have a clearer picture, but after testing a few, it seems the interoperability is hit-and-miss and rather problematic for pre-D3D8 games.

EDIT2:

If none of the above, sure, I can use one-off exes with vsync disabled.

eddman wrote on Yesterday, 20:38:

I already managed to do it for D3D8 by combining dgVoodoo with Special K, but am having a lot of trouble getting that combo to work with D3D7 and older.

Tbh, I can't see any reason why they don't work with SpecialK. I don't really know SpecialK but a dgVoodoo-wrapped D3D7 game is (should) be the very same DX11/12 game as a D3D8 one from SK's viewpoint.
There is no difference in the implementation of the base layers (like the backend API initialization, the image presenters, etc.) of D3D7 and 8, in fact they share a common code base.
I think it'll be easier for now to give you a vsync-free x86 build from the DX dll's (2.87.3).