Convert VSDX to VDX

It appears that the VDX format was abandoned in Visio 2013 and since that version the program no longer can save in the format. If you really need to create an .vdx file from .vsdx file, your only option would be to use one of the earlier version of Visio version, something like like 2010. 3rd party software for this conversion is also limited, as Visio formats are rarely supported in programs other than the ones from Office.