The GD.vxf field is a local cache of the the number of fractional bits set in the VERTEX_FORMAT, if I understand it correctly. It is initialized to 4 in GDClass::begin but does not appear to be updated elsewhere in the code. The GD.vxf field is used by the Bitmap and Poly classes.
Should it be updated in GDClass::VertexFormat? It looks like it may also be necessary in GDClass::cmd_dlstart, though I can't seem to find documentation to support that in the FT81x Series Programmer Guide.