the .ply files are in MNI space.
You’d need to convert the coordinates to native space e.g. using ea_map_coords (but it involves some programming in Matlab).
Finally, visualizing VTAs – you’d need to convert the .nii files exported by Lead-DBS to surfaces – e.g. using isosurface() – and also export these as .PLY. These are in MNI space, though, too.
It’s all a bit involved and does require programming custom code + good knowledge about coordinate systems of nifti files and nonlinear warps.
If you prefer the 3D Slicer Render Engine over Matlab, one alternative idea could also to do the whole processing in Slicer instead of Lead-DBS? I know that the pyDBS software developed in Rennes / Paris is able to run inside 3D Slicer.
Hope this helps!