If you encounter a problem of keyboard mapping when you use the vncviewer,
the solution is to add "export XKL_XMODMAP_DISABLE=1" to the file "~/.vnc/xstartup" on the server which looks like:
xsetroot -solid grey
#x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
Of course, you have to restart xserver to implement the changes.