I think @K0bin is already aware of this, but I've recently come across it in d3d8, so I'll mention that in D3DCREATE_MIXED_VERTEXPROCESSING mode apps can switch between HWVP (the default) and SWVP at any time by calling SetSoftwareVertexProcessing() in d3d9, or by setting the D3DRS_SOFTWAREVERTEXPROCESSING render state in d3d8. This scenario should be handled as well.
Just a reminder issue for https://github.com/doitsujin/dxvk/pull/3248#issuecomment-1448097202 to be evaluated so it's not forgotten.