Place the following as .htaccess in your forum root
Code:
RewriteEngine On
RewriteRule forumdisplay/(.*) http://www.yourhost.com/forum/archive/index.php/$1 [R]
RewriteRule showthread/(.*) http://www.yourhost.com/forum/archive/index.php/$1 [R]
Please note that http://www.yourhost.com/forum must be changed to the real URL
This should make your already indexed vBArchive URLs compatible with the built-in archive system of vB3.