We have found one software suitable in our database for this conversion.
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.
Updated: July 15, 2016
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.