I did not explain myself properly,
the portal is installed in /portal/
the phpbb is installed in /forum/
the problem is that the portal uses relative links.
In the original installation of the forum, it will *always* be (except the admin) in the root directory of /forum/, the portal considers this and creates relative links (fixed) as ../portal/whatever_directory/.
So my problem is that when a "fake" or "virtual" subdirectory is created with the rewrite, the portal STILL creates relative links as ../portal/ but it does not find, since the browser will go search just one directory up, and will so go in /forum/portal. I thought about creating a symlink, but that would only be a temporary solution so a rewrite rule (i guess) could help me much better by redirecting all ../portal/ calls to the correct dir.
Hope i explained myself better this time