As far as we know, vssx to vsdx conversion is only possible using Microsoft Visio and represent transformation or using of stencil (.vssx) to drawings (.vsdx) file. Visio formats are notoriously ill-supported in other programs, so the original MS Visio remains your best option.
Convert Microsoft Visio stencil file to Microsoft Visio drawing.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert vssx to vsdx on Windows
A vector based diagramming software from Microsoft
Convert from vssx file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.