URL Rewrite from table

[ilanur]

hello, it's my first post here, thanx to everyone reading this and sorry for my english.
Here is my situation:
I imported my old forum with impex ( joomlaboard) and everything went fine.
the problem is that i need to keep my old url for every single thread in the forum, and the url of the old forum didn't follow a specific regular expression.
i made a table in sql with the new id ( the one given by vbulletin) and the oldurl, so i have a "link" beetween the new url and the old one.
then i modified my htaccess this way:

RewriteEngine On
RewriteCond %{QUERY_STRING} t=([0-9]+)
RewriteRule showthread\.php\? redirect.php?action=thread&vbid=%1 [L]

With this i didn't solve the problem because when i click on a thread it opens the correct page ( www.example/oldurl.html) but it gives the 404 not found error. And when i pass my mouse on the thread ( without clicking on it) i see that the real link is still to the new url ( /showthread.php?=id).. so i understand that rewriterule doesn't really rewrite urls but just redirect them....
any help on solving this? i also need a solution compatible with vbseo ( for the future threads) or any other seo plugin (which is the best?)
thank you, best regards, ilanur.