Belgian cuisine is not very friendly to vegetarians, let alone to vegans. In most restaurants you'll have a hard time finding a vegan option, Asian restaurants being the usual exception (but watch out for fish sauce).
In most supermarkets you can find rice milk.
Soy is fairly common, also organic soy.
If you're into chocolate you should check out the Cote d'Or pure chocolate spread. It's vegan and pretty awesome.