It seems that latest versions of Microsoft Visio no longer support formats from older versions. This means you will have to use old versions of Visio if you need to save your diagrams or flowcharts to some previously used format. That pretty much also goes for vdx to vst conversion.
Convert Microsoft Visio drawing XML file to Microsoft Visio flowchart.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert vdx to vst on Windows
A diagram application, that uses vector graphics to create diagrams. Microsoft Visio is distributed as part of Microsoft Office Family and also available as a subscription through Microsoft Office 365. The latest version is Visio 2016 that is compatible with Windows 7, Windows 8 and Windows 10.
Visio allows users to create professional diagrams in just few clicks. Visio makes diagramming simple—whether you want to quickly capture a flowchart that you brainstormed on a whiteboard, map an IT network, build an organizational chart, document a business process, or draw a floor plan.
Convert from vdx file
to vst file
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.