I have tried to compile and install xfce4-vala without success, following the steps detailed below.
Once the repository is cloned, I go inside and execute:

   $ sh autogen.sh

Output:
   Preparing package directory /home/<my_user>/Proyectos/XFCE/xfce4-vala...
   Running aclocal   -I /usr/share/xfce4/dev-tools/m4macros -I /usr/share/xfce4/dev-tools/m4macros...
   Running automake --force-missing --add-missing --copy --gnu...
   Running autoconf...

   Running /home/<my_user>/Proyectos/XFCE/xfce4-vala/configure --enable-maintainer-mode ...
   checking for a BSD-compatible install... /usr/bin/install -c
   checking whether build environment is sane... yes
   checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
   checking for gawk... gawk
   checking whether make sets $(MAKE)... yes
   checking whether make supports nested variables... yes
   checking whether to enable maintainer-specific portions of Makefiles... yes
   checking for pkg-config... /usr/bin/pkg-config
   checking pkg-config is at least version 0.9.0... yes
   configure: WARNING: 
   configure: WARNING: The package garcon-2 >= 0.4.0 was not found
   configure: WARNING: You need it to run the introspection tool
   configure: WARNING: 
   checking for VALA... no
   configure: error: Package requirements (libvala-0.34) were not met:

   Package 'libvala-0.34', required by 'virtual:world', not found

   Consider adjusting the PKG_CONFIG_PATH environment variable if you
   installed software in a non-standard prefix.

   Alternatively, you may set the environment variables VALA_CFLAGS
   and VALA_LIBS to avoid the need to call pkg-config.
   See the pkg-config man page for more details.


Apparently it is a problem in the Vala version, so I have checked that everything is up to date.

   $ vala --version

Output:
   Vala 0.48.5


   $ ls /usr/lib | grep '^libvala-.*'

Output:
   libvala-0.48.so
   libvala-0.48.so.0
   libvala-0.48.so.0.0.0


I have also tried installing it from the AUR but got the same error. My distribution is Arch Linux.