Thank you for contacting support. Unfortunately, there is no way to read from VSTX template. It is not a supported format. We’ve logged a feature request under ticket id DIAGRAMJAVA-50266 in our issue tracking system. We’ll let you know once it is supported. We’re sorry for the inconvenience caused.
Thank you for being patient. We have a good news for you that the issue id DIAGRAMJAVA-50266 has now been resolved. If there is no issue in the QA phase, then this fix will be included in the next version of Aspose.Diagram for Java 5.9.0. We’ll inform you via this forum thread as soon as the new release is published.