[chikuru]

At the moment, the forum is in vbulletin 4 and it is installed in /forum directory.
I am planning to upgrade it to vBulletin 5 and move the installation to the / directory.
How can I create 301 redirect without double 301 redirect?

The vb4 link will be redirected to vb5 using the vb system, then redirect it to equivalent page in / directory

[In Omnibus]

There is absolutely no reason to do this.

You have to delete the vBulletin 4 folders and files to install vBulletin 5.

vBulletin 5 automatically creates redirects for vBulletin 4 upgrades unless you have screwed up your vBulletin 4 database with third party SEO modifications.