Glade generates a UI description in an XML format. GTK contains a class called Gtk.Builder
which reads this XML and constructs a widget hierarchy.
"Traditionally", the way to tie the two together is to give the widgets meaningful names in Glade, and then use widget = Gtk.Builder.get_object(name)
to grab the widget for use in your programme. You'll find quite a few examples like this on the web (google "vala gtkbuilder" or something similar to find them).
However, the latest versions of Vala and GTK have some new features to cut out quite a bit of the boilerplate from the way things have been done in the past. It's very new so you won't find many examples yet, but this blog post has all the details:
http://blogs.gnome.org/tvb/2013/05/29/composite-templates-lands-in-vala/