2.1.1738 Part 4 Section 8.1, VML Drawing Part
a. The standard implies that the VML Drawing part is not an XML part but contains XML-style markup.
In Office, the VML Drawing part closely resembles XML. An XML parser designed to read and write VML ("[ISO/IEC-29500-4] §19.1; VML") markup can parse the contents of a VML Drawing part by consuming the XML root element and then handling the contained VML markup normally.