I had a look and to me the site is actually loaded over HTTPS but there are third party resources such as images and other embedded content that are loaded over HTTP which makes browsers show that the connection is not secure (even though HTTPS is still present in the URL)
You can try this plugin
https://vborg.vbsupport.ru/showthread.php?t=288060 which is for vBulletin 4, but it has a comment that explains how to make it work on vBulletin 3 at
https://vborg.vbsupport.ru/showthrea...87#post2411587
You should also add
HTML Code:
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">
to the header.