There is already a bug report for XML includes #360067.
but my question is, is there a workaround?
Can I create with Vex multiple documents that do not start
with the root element of "book" and link them together in
Right now when I open a file that includes an xi:include element
Vex seems quite confused, that is I cannot edit the document
any further (the list of elements in "add element ..." is empty).
currently it is not possible to insert the <xi:include> element with Vex if it is not defined in the DTD/Schema of the document type you are using. As a workaround you could make a copy of the DocBook DTD/Schema and add xi:include at the positions where you want to use it. To put your DTD/Schema in charge, you also have to modify the XML catalog in the Eclipse Preferences to point to your version instead of the version that comes with Vex.
Vex should be able to handle other root elements than book. If you use the New Document wizard, you can choose other root elements, for example. If it does not work for your document, please file a bug and attach an example document.