There is only one file type that uses the .VSTX extension:
Visio Drawing Template
Drawing template created by Visio, a technical drawing program; provides a starting point for making other drawings (.VSDX files) from the same starting conditions and with the same look and feel.
NOTE: The VLTX and VSDX formats were introduced with Visio 2013. Prior versions of the software use .VTX files for templates.
Updated: June 25, 2013