I should explain myself, sorry to be so terse, Bruno.
This change is simply doing what VASMCC should do. It will work for any keyboard on the VASM list under Hardware > Keyboard Map Selection.
To get your keyboard up you will have edit the file /etc/X11/xorg.conf as root. I suspect this will be difficult if you don't have access to a US-layout keyboard.
I don't know how familiar you are with the console but it goes like this:
Open the terminal or console
(put in your root password)
Then copy and paste the following line (copy normally but paste at the command line with Shift+Ctrl+V to minimise your typing)
Once the editor opens search down until you come to the line in the Keyboard section:
Option "XkbLayout" "us"
change it to
Option "XkbLayout" "be"
(just change those two letters)
Save the file and reboot for the settings to take effect.
Your keyboard will have the Belgian layout when you login.
I notice there are two types of Belgian keyboard so maybe, if it isn't right the first time, you will need repeat the process and make the line look like:
Option "XkbLayout" "be2"