By default, you can install vBulletin to make every URL on the forum https. However, this does not mean that upon viewing your website or typing your domain into the address bar on a clients web browser, you will be forced or directed to a secure connection. In fact, that first page will be insecure, and only the next page they navigate to and further will be secured by SSL.

What you need to do is add something like this to your .htaccess file that is located in the root directory of your vBulletin installation.

Code:
RewriteEngine On 
RewriteCond %{SERVER_PORT} 80
RewriteCond %{HTTP_HOST} ^overclockedlan\.pro$ [OR]
RewriteCond %{HTTP_HOST} ^www\.overclockedlan\.pro$
RewriteRule ^(.*)$ https://overclockedlan.pro/$1 [R,L]
Obviously, you want to replaced "overclockedlan" and ".pro" with your domains information. The "https://" should remain left alone.
You should already have an active, installed SSL certificate before you attempt to do this, as it will cause your browser to go haywire.

Upon success, when first visiting your website by typing the domain in your address bar without https://

e.g overclockedlan.pro

You should be presented with a secure, encrypted page provided there are no un-encrypted images on the page.

All images and embed codes should have http:// replaced in them to https:// to ensure maximum security across your website.

You must do it manually with another plugin, or edit everyone's posts. If you don't, your SSL will break in between pages.

If you would like to automagically have all images and embed codes converted to https:// URL links, you can install this plugin:
IMG Cacher - SSL Keeper - Mixed Content Block Solution by MegaManSec
https://www.vbulletin.org/forum/showthread.php?t=288060

While this plugin was designed for vBulletin version 4.1, it is working on 4.2.x no problem it seems.