If you want to use the GTK+ composite templates in Vala, please remember: you have to use a GResource. The syntax is pretty simple, here’s an example:

<?xml version="1.0" encoding="UTF-8"?>
  <gresources>
    <gresource prefix="/org/gtk/Example">
      <file compressed="true">window.ui</file>
    </gresource>
  </gresources>

Let’s say you called that file project.gresource.xml, to use it in your project you need to pass a gresource at compile time:

valac --target-glib=2.38 --gresources project.gresource.xml --pkg your_package1 --pkg you_package2 ...

Of course, it can’t be that simple, so first you have to compile that xml file (crazy, isn’t it?), doing glib-compile-resources project.gresource.xml and then include the generated C file. As this won’t scale, and you probably aren’t compiling without a build system, the documentation suggest, in the case of make, to include a rule, which looks like this:

project-gresource.c: project.gresource.xml $(shell $(GLIB_COMPILE_RESOURCES) --generate-dependencies project.gresource.xml)

and we can include the generated C file in our valac call:

valac --target-glib=2.38 --gresources project.gresource.xml --pkg your_package1 --pkg you_package2 file1.vala file2.vala project-gresource.c ...

Please don’t forget to read the announcement here for more info.