The location (coordinates) for the vertices are always saved as SCS.
The are found in the sHeadModel.GridLoc
The coordinate conversion is failing as the fist argument in the cs_convert() must be the sMRI structure, not the head model. It can be retrieved as:
sSubject = bst_get('SurfaceFile', sHeadModel.SurfaceFile);
sMri = in_mri_bst(sSubject.Anatomy(sSubject.iAnatomy).FileName);