Where is the redirection performed?

[ndoktoruser]

Hello all,

I have migrated my forum to vBulletin 4, then upgraded it to vBulletin 5.
In order to not lose the visitors who come from external links, I created a table with a redirection mapping from the old links to the new ones.

So where is redirection treatment performed in vBulletin 5?
I suppose the system tries to find the page related to the URL, (in case it doesn't) then it tries to match a redirection from vBulletin 4, (in case it doesn't) then it leads the user to a 404 page. However, I want to put some treatment before the 404, in the case I can find out it is a link from the old system, so I would redirect to the proper vBulletin 5 URL.

Thanks in advance!