2.1.37 Part 1 Section 15.2.17, 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 ECMA-376 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.