Try this one:
Attachment 61703

Upload it into your /forum/ folder, it will include the vB files from the root directory and redirect there, it's customised just for your situation.

Hey, this is an excellent script, i had something like this for my old blog :) you saved my time :D

Does exactly as what it says on the packet. Nice.

good if you just SEO'ed

i need to see a demo

This would set 301 redirects for the search engines.

header( "HTTP/1.1 301 Moved Permanently" );
header("Location: ".$redir);
exit();

There is something not clear for me , i had a phpBB2 forum which is reside in folder1 and i moved to VBulletin in another folder let us say folder2 , i imported all the data using Impex tool , so i want to confirm this viewtopic.php it should be copied in the old forum folder (folder1) ?

do not working on my 3.66.
redirecting to forumhome page

this is great, what about those who used mod_rewrite on phpBB2 ? SEO Mod

Meaning how do i get this url http://www.alizee-forum.com/topic,14063,.html redirected properly?

Meaning to get it to redirect to http://www.alizee-forum.com/viewtopic.php?t=14063 and ur script will then do redirect to http://www.alizee-forum.com/showthread.php?t=16946

There's a working demo in the main post, but I'll repost it here:
Working demo
Thread name "The Apogee Interview series and the weekly Prey update"
Old phpBB2 URL: http://www.bluesbrotherscentral.com/...pic.php?t=2374
New vB URL: http://www.bluesbrotherscentral.com/...ead.php?t=2190

Thanks for that, I can't believe I didn't think of including that when I've used it everywhere else on my site! Added it into the code, released v1.01, and put your name in the credits :)

Your old forum used to be in /folder1/viewtopic.php?t=1234
Your new forum is in /folder2/showthread.php?t=5678
What you're doing is replacing the original viewtopic.php page with my file. Now, you're going to need to modify the viewtopic.php code slightly so it includes the config.php file in folder2.
Line 16 should become:
Line 60 should become:
I think that's all you need to do, but I haven't tested it.

If it can't get the topic ID or the post ID, then it redirects back to the homepage.
You need to make sure that when you ran Impex it kept all the importpostid and importthreadid values in the vb_thread and vb_post tables. Redirecting to the homepage is the failsafe for threads which have been deleted or invalid links.

You're going to have to find a way to do that yourself, I would think you could just copy whatever you were doing for phpBB2. If you used .htaccess, add the same thing to your current .htaccess file.
Just as a guess, maybe:
(I genuinely have no idea if that will work, found the snippet on another site)
This should redirect back to /viewtopic.php, which will then check your vBulletin tables for the new Thread or Post ID, and do the redirect as normal.
It's 3 steps instead of 2, but not a huge problem :)