The changes since Vala 0.20.1 follows:
- Support \uXXXX escape sequences.
- Support specifying vfunc order in interfaces to define ABI.
- Support GTK+ widget templates.
- Bug fixes and binding updates.
For a primer on how to use GTK+ widget templates, please see: http://blogs.gnome.org/tvb/2013/05/29/composite-templates-lands-in-vala/
The above tutorial is not up-to-date but it's a good starting point.
About specifying vfunc order, it's done with [CCode (ordering = x)] where x is the desired position of the method in the vtable. Either none or all of the virtual functions must have an ordering.