Both .vst and .vss files come from Microsoft Visio but are used for a bit of different content. While it is certainly possible to reuse various elements used in Visio, direct conversion is not always possible, especially on file per file basis. And this might actually apply to vst to vss conversion.
If you struggle with export of Visio files or have some other issue, it might be best to reach out to Microsoft support / community or consult help files in Visio.
Updated: October 10, 2023