When one is using pretty/SEO friendly URLs in vBulletin, then old/default thread URLs will be redirected to new pretty URLs.
vBulletin plugin like XML Sitemaps is generating sitemap with default URLs so Google will not index redirects.

So how to change redirect type?

You can easy change the redirect type by editing the includes/class_friendly_url.php file which is located under your vBulletin folder.

The following lines should be changed:

from:
$code = 302;
to:
$code = 301;

and


from:

exec_header_redirect($url, 302);


to:

exec_header_redirect($url, 301);

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In my case i have following there:
$url = $this->get_url(false, true);
$code = 301;