Doesn’t this add on come with rewrite rules to rewrite forums/showthread.php to the right file that came with the add on?
The manual says that if XenForo's URL path is the same as vB's, then there is nothing extra needed to make the addon work.
If they are not the same path, then there is extra work required where you create the directory structure and copy some files there so XenForo can pick up the requested path and process it.
xenforo.com
In our case, the URL paths are the same (/forums/ for vB and /forums/ for XenForo) but the actual web root directory paths are not the same (the vBulletin instance did use a /forums/ directory whereas XenForo does not and that path is routed internally). I don't know if this distinction matters. If I try to add a /forums/ directory, that causes native XenForo URLs to break (the site essentially becomes unusable and returns 404s everywhere), likely due to the way nginx handles try_files.
At any rate, the addon does not appear to be able to redirect any URLs regardless of the form in which they are used. I just have no troubleshooting leads to go on as to what is wrong to even guess at how to fix it. The addon is supposed to be plug and play.
We had URL redirection working back on XenForo 1. I don't remember any details of that setup but I know it was totally different.