What is the best approach to redirect sub.mysite.com/forum/ to sub.mysite.com?

If you’re using Apache, you can use Rewrite rules to rewrite sub.example.com/forum to sub.example.com.

You can add following to your Virtual Host or .htaccess for sub.example.com:

RewriteEngine on 
RewriteRule ^forum/(.*)$ $1

If you changed via Virtual Host make sure restart Apache after change. If you go with .htaccess, make sure it’s enabled. This will ‘just remove’ forum/ part from your link.

If you decide to go with Virtual Host change, it will look something like:

<VirtualHost *:80>
    ServerName sub.example.com
    RewriteEngine on 
    RewriteRule ^forum/(.*)$ $1
    . . .
</VirtualHost>

You can find virtual hosts files in /etc/apache2/sites-available. Default file is 000-default.conf.

You can also go with redirection, but rewrite rules should be more suitable.

If you use nginx, I can write down instructions too, if needed.