Ok, it seams thats another problem because virtualdomains works fine, but for a reason I can't explain the menu translation on the startpage doesn't work.
if you go in english to a sub site and change the language it works fine (incl. the Menu). I think a Plugin or Module (that I didn't found) intercepts the translation process or caches the content of the startpage....
virtualdomains doesn't remove the /nl/ prefix I'm not sure if this is possible without rewriteing the joomle language filter plugin.