why we can not import .vss stencils to libredraw, or why there no useful stencils added to draw ,
I think Libreoffice is not serious about it .
The developer comment in tdf#60639#c4 would seem to indicate why. The EMF wrapper is handled via tools that are external to LO (part of the librevenge infrastructure library). Refer this blog post on how to build the required vss2odg tool.