There is only one file type that uses the .VSSX extension:
Visio Stencil File
A VSSX file contains stencils created by Microsoft Visio, a technical drawing program. It stores a collection of shapes, connectors, or other objects that can be used within a drawing. VSSX files are often used by organizations for distributing logos and other standard design elements that can be included in technical drawings.
You can open, edit, and save VSSX files with Microsoft Visio (Windows). To add the shapes in a VSSX file to your installation of Visio, simply double-click the file and Visio will import the shapes into your stencils library.
You can also open VSSX files with Nevron Software MyDraw (multiplatform) and The Omni Group OmniGraffle (macOS).
Updated: April 6, 2021