I've tried to input digraphs in Vim (without vi compatibility), and unfortunately I can't. After using ControlKP * it should input big greek pi letter. I've also tried using PBS * with :set digraph, but no success either.

My gvim --version output:

VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Oct 23 2012 18:42:18)
Included patches: 1-712
Compiled by ArchLinux
Big version with GTK2 GUI.  Features included (+) or not (-):
+arabic +autocmd +balloon_eval +browse ++builtin_terms +byte_offset +cindent 
+clientserver +clipboard +cmdline_compl +cmdline_hist +cmdline_info +comments 
+conceal +cryptv +cscope +cursorbind +cursorshape +dialog_con_gui +diff 
+digraphs +dnd -ebcdic +emacs_tags +eval +ex_extra +extra_search +farsi 
+file_in_path +find_in_path +float +folding -footer +fork() +gettext 
-hangul_input +iconv +insert_expand +jumplist +keymap +langmap +libcall 
+linebreak +lispindent +listcmds +localmap +lua +menu +mksession +modify_fname 
+mouse +mouseshape +mouse_dec +mouse_gpm -mouse_jsbterm +mouse_netterm 
+mouse_sgr -mouse_sysmouse +mouse_urxvt +mouse_xterm +multi_byte +multi_lang 
-mzscheme +netbeans_intg +path_extra +perl +persistent_undo +postscript 
+printer -profile +python -python3 +quickfix +reltime +rightleft +ruby 
+scrollbind +signs +smartindent -sniff +startuptime +statusline -sun_workshop 
+syntax +tag_binary +tag_old_static -tag_any_white -tcl +terminfo +termresponse
 +textobjects +title +toolbar +user_commands +vertsplit +virtualedit +visual 
+visualextra +viminfo +vreplace +wildignore +wildmenu +windows +writebackup 
+X11 -xfontset +xim +xsmp_interact +xterm_clipboard -xterm_save 
   system vimrc file: "/etc/vimrc"
     user vimrc file: "$HOME/.vimrc"
      user exrc file: "$HOME/.exrc"
  system gvimrc file: "/etc/gvimrc"
    user gvimrc file: "$HOME/.gvimrc"
    system menu file: "$VIMRUNTIME/menu.vim"
  fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_GTK  -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng15   -I/usr/local/include  -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4  -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1      
Linking: gcc   -L. -Wl,-O1,--sort-common,--as-needed,-z,relro -rdynamic -Wl,-export-dynamic  -Wl,-E -Wl,-rpath,/usr/lib/perl5/core_perl/CORE  -Wl,-O1,--sort-common,--as-needed,-z,relro -L/usr/local/lib -Wl,--as-needed -o vim   -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lpangocairo-1.0 -lgdk_pixbuf-2.0 -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 -lglib-2.0   -lSM -lICE -lXt -lX11 -lXdmcp -lSM -lICE -lm -lncurses -lnsl   -lacl -lattr -lgpm -ldl  -L/usr/lib -llua  -Wl,-E -Wl,-rpath,/usr/lib/perl5/core_perl/CORE -Wl,-O1,--sort-common,--as-needed,-z,relro -fstack-protector -L/usr/local/lib  -L/usr/lib/perl5/core_perl/CORE -lperl -lnsl -ldl -lm -lcrypt -lutil -lpthread -lc -L/usr/lib/python2.7/config -lpython2.7 -lpthread -ldl -lutil -lm -Xlinker -export-dynamic   -lruby -lpthread -lrt -ldl -lcrypt -lm  -L/usr/lib 

Can you tell me how to get my ControlK digraphs work?

EDIT:

Output of :verbose set enc? fenc? is:

encoding=utf-8
  Last set from ~/.vimrc
fileencoding=