Sorry, I don't have time to lookup the exactly syntax, and I don't have all these things memorized. I can get you headed in the right direction though.
First off, I can tell you the .htaccess file will need to be in the root directory, not the vb directory.
You will then need to make a rewriterule for each page. I believe this will work:
RewriteRule showthread.php /vb/showthread.php [QSA,R=301]
If my memory is correct, this will work. The first argument (showthread.php) is what you are checking the URL against. If it matches, you send them to the second argument (/vb/showthread.php).
Lastly, you have two flags added to the end. The QSA stands for 'query string attached' and it should ensure everything after the question mark gets passed along. The second flag, R=301 tells the browser to do a permanent redirect to the new location.
Again, all this advice is out of memory, so I'm not 100% sure it will work. If not, let me know and I'll look up a proper solution when I have the time.